/* Sharpen — styles. Tokens first, then base, layout, components, views. */

:root {
  --page: #F2F4F8;
  --surface: #FFFFFF;
  --sunken: #E9EDF3;
  --ink: #0F1728;
  --ink-2: #2A3446;
  --muted: #5B6577;
  --line: #DCE1EA;
  --line-strong: #C7CFDC;
  --accent: #2B55F5;
  --accent-ink: #2448D8;
  --accent-soft: #E7ECFE;
  --accent-hover: #2149E0;
  --on-accent: #FFFFFF;
  --solved: #0E9F6E;
  --solved-ink: #0A7D57;
  --solved-soft: #E2F4EC;
  --warn: #B7791F;
  --warn-ink: #8A5A12;
  --warn-soft: #FBF1DE;
  --pitfall: #D1435B;
  --pitfall-ink: #B32D45;
  --pitfall-soft: #FBE8EB;
  --lvl-middle: #0E8A8A;
  --lvl-middle-ink: #0B7474;
  --lvl-middle-soft: #DFF1F1;
  --lvl-high: #2B55F5;
  --lvl-high-ink: #2448D8;
  --lvl-high-soft: #E7ECFE;
  --lvl-college: #E2572B;
  --lvl-college-ink: #BF4419;
  --lvl-college-soft: #FCE9E2;
  --s1: #2B55F5;
  --s2: #E2572B;
  --s3: #0E8A8A;
  --s4: #0E9F6E;
  --s5: #B7791F;
  --s6: #D1435B;
  --plot-grid: #E6EAF0;
  --plot-grid-minor: #F0F2F6;
  --plot-axis: #7A8497;
  --focus-ring: #2B55F5;
  --shadow: 0 1px 2px rgba(15, 23, 40, 0.04), 0 6px 20px rgba(15, 23, 40, 0.06);

  --f-head: 'Bricolage Grotesque', 'Figtree', ui-sans-serif, system-ui, -apple-system, 'Segoe UI', sans-serif;
  --f-body: 'Figtree', ui-sans-serif, system-ui, -apple-system, 'Segoe UI', sans-serif;
  --f-mono: 'JetBrains Mono', ui-monospace, 'SFMono-Regular', Menlo, Consolas, monospace;

  --r-card: 20px;
  --r-ctl: 12px;
  --r-inner: 14px;
  --wrap: 1080px;
  --read: 720px;
  --gutter: 16px;
  --header-h: 60px;

  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --page: #0C111B;
    --surface: #141B29;
    --sunken: #0F1521;
    --ink: #E8ECF4;
    --ink-2: #CDD3E0;
    --muted: #96A0B5;
    --line: #243047;
    --line-strong: #33415C;
    --accent: #7C9BFF;
    --accent-ink: #9DB4FF;
    --accent-soft: #1B2747;
    --accent-hover: #98B0FF;
    --on-accent: #0C111B;
    --solved: #3CC79A;
    --solved-ink: #5ED6AE;
    --solved-soft: #12302A;
    --warn: #E0A84A;
    --warn-ink: #EDBE6C;
    --warn-soft: #2F2615;
    --pitfall: #F0758A;
    --pitfall-ink: #F59AAA;
    --pitfall-soft: #34191F;
    --lvl-middle: #3CC4C4;
    --lvl-middle-ink: #6AD6D6;
    --lvl-middle-soft: #12302F;
    --lvl-high: #7C9BFF;
    --lvl-high-ink: #9DB4FF;
    --lvl-high-soft: #1B2747;
    --lvl-college: #FF8A5C;
    --lvl-college-ink: #FFA581;
    --lvl-college-soft: #36201A;
    --s1: #7C9BFF;
    --s2: #FF8A5C;
    --s3: #3CC4C4;
    --s4: #3CC79A;
    --s5: #E0A84A;
    --s6: #F0758A;
    --plot-grid: #1F293C;
    --plot-grid-minor: #182133;
    --plot-axis: #7D879C;
    --focus-ring: #7C9BFF;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 8px 24px rgba(0, 0, 0, 0.28);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --page: #0C111B;
  --surface: #141B29;
  --sunken: #0F1521;
  --ink: #E8ECF4;
  --ink-2: #CDD3E0;
  --muted: #96A0B5;
  --line: #243047;
  --line-strong: #33415C;
  --accent: #7C9BFF;
  --accent-ink: #9DB4FF;
  --accent-soft: #1B2747;
  --accent-hover: #98B0FF;
  --on-accent: #0C111B;
  --solved: #3CC79A;
  --solved-ink: #5ED6AE;
  --solved-soft: #12302A;
  --warn: #E0A84A;
  --warn-ink: #EDBE6C;
  --warn-soft: #2F2615;
  --pitfall: #F0758A;
  --pitfall-ink: #F59AAA;
  --pitfall-soft: #34191F;
  --lvl-middle: #3CC4C4;
  --lvl-middle-ink: #6AD6D6;
  --lvl-middle-soft: #12302F;
  --lvl-high: #7C9BFF;
  --lvl-high-ink: #9DB4FF;
  --lvl-high-soft: #1B2747;
  --lvl-college: #FF8A5C;
  --lvl-college-ink: #FFA581;
  --lvl-college-soft: #36201A;
  --s1: #7C9BFF;
  --s2: #FF8A5C;
  --s3: #3CC4C4;
  --s4: #3CC79A;
  --s5: #E0A84A;
  --s6: #F0758A;
  --plot-grid: #1F293C;
  --plot-grid-minor: #182133;
  --plot-axis: #7D879C;
  --focus-ring: #7C9BFF;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 8px 24px rgba(0, 0, 0, 0.28);
  color-scheme: dark;
}

/* ---------- base ---------- */
*, *::before, *::after { box-sizing: border-box; }
[hidden] { display: none !important; }
html { -webkit-text-size-adjust: 100%; text-size-adjust: 100%; scroll-padding-top: calc(var(--header-h) + 16px); }
body {
  margin: 0;
  background: var(--page);
  color: var(--ink);
  font-family: var(--f-body);
  font-size: 16px;
  line-height: 1.6;
  font-feature-settings: 'ss01' 0;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  overflow-x: hidden;
}
body.has-focus { overflow: hidden; }
main { flex: 1; outline: none; }
h1, h2, h3 { font-family: var(--f-head); font-weight: 700; line-height: 1.15; margin: 0; letter-spacing: -0.015em; color: var(--ink); }
h1 { font-size: clamp(28px, 4.6vw, 42px); letter-spacing: -0.025em; }
h2 { font-size: 22px; font-weight: 600; }
h3 { font-size: 18px; font-weight: 600; }
p { margin: 0; }
a { color: var(--accent-ink); text-decoration-thickness: 1px; text-underline-offset: 3px; }
strong { font-weight: 600; }
.mono { font-family: var(--f-mono); font-size: 12.5px; letter-spacing: 0.01em; font-variant-numeric: tabular-nums; }
.muted { color: var(--muted); }
.overline {
  font-family: var(--f-mono); font-size: 11.5px; font-weight: 500; letter-spacing: 0.06em; text-transform: uppercase;
  color: var(--muted); margin: 0;
}
.visually-hidden {
  position: absolute !important; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden;
  clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}
.icon { flex: none; display: inline-block; vertical-align: middle; }
:focus-visible { outline: 2px solid var(--focus-ring); outline-offset: 2px; border-radius: 6px; }
::selection { background: var(--accent-soft); color: var(--ink); }

.wrap { width: 100%; max-width: calc(var(--wrap) + 2 * var(--gutter)); margin: 0 auto; padding: 0 var(--gutter); }
@media (min-width: 720px) { :root { --gutter: 28px; } }

.skip-link {
  position: absolute; left: 12px; top: -60px; z-index: 100; background: var(--surface); color: var(--ink);
  padding: 8px 14px; border-radius: var(--r-ctl); border: 1px solid var(--line); text-decoration: none;
}
.skip-link:focus { top: 10px; }

/* ---------- header / footer ---------- */
.site-header {
  position: sticky; top: 0; z-index: 20;
  background: var(--page);
  border-bottom: 1px solid var(--line);
}
.header-in { display: flex; align-items: center; justify-content: space-between; height: var(--header-h); gap: 12px; }
.brand {
  display: inline-flex; align-items: center; gap: 9px; text-decoration: none; color: var(--ink);
  font-family: var(--f-head); font-weight: 700; font-size: 20px; letter-spacing: -0.02em;
}
.brand-mark { color: var(--accent); }
.site-nav { display: flex; align-items: center; gap: 4px; }
.site-nav a {
  color: var(--muted); text-decoration: none; font-weight: 500; font-size: 15px;
  padding: 7px 12px; border-radius: var(--r-ctl);
}
.site-nav a:hover { color: var(--ink); background: var(--sunken); }
.site-nav a[aria-current="page"] { color: var(--ink); background: var(--surface); box-shadow: inset 0 0 0 1px var(--line); }
.site-footer { margin-top: 72px; border-top: 1px solid var(--line); }
.footer-in { display: flex; justify-content: space-between; gap: 12px; flex-wrap: wrap; padding-top: 20px; padding-bottom: 28px; font-size: 13.5px; }
.footer-in > span:first-child { font-family: var(--f-head); font-weight: 600; }

/* ---------- buttons ---------- */
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 7px;
  font: inherit; font-weight: 600; font-size: 15px; line-height: 1;
  height: 42px; padding: 0 16px; border-radius: var(--r-ctl);
  border: 1px solid var(--line); background: var(--surface); color: var(--ink);
  cursor: pointer; text-decoration: none; white-space: nowrap;
  transition: background-color .15s, border-color .15s, color .15s;
}
.btn:hover { border-color: var(--line-strong); background: var(--sunken); }
.btn:disabled { opacity: .5; cursor: not-allowed; }
.btn-sm { height: 36px; padding: 0 12px; font-size: 14px; border-radius: 11px; }
.btn-primary { background: var(--accent); border-color: var(--accent); color: var(--on-accent); }
.btn-primary:hover { background: var(--accent-hover); border-color: var(--accent-hover); }
.btn-primary:disabled:hover { background: var(--accent); }
.btn-ghost { background: transparent; }
.btn-icon { width: 40px; height: 40px; padding: 0; background: transparent; border-color: transparent; color: var(--muted); }
.btn-icon:hover { color: var(--ink); background: var(--sunken); border-color: transparent; }
.btn.is-armed { border-color: var(--pitfall); color: var(--pitfall-ink); background: var(--pitfall-soft); }

/* chip */
.chip {
  display: inline-flex; align-items: center; height: 26px; padding: 0 11px; border-radius: 999px;
  font-family: var(--f-mono); font-size: 11.5px; font-weight: 500; letter-spacing: 0.04em; text-transform: uppercase;
  background: var(--lvl-soft, var(--accent-soft)); color: var(--lvl-ink, var(--accent-ink));
}
.lvl-middle { --lvl: var(--lvl-middle); --lvl-ink: var(--lvl-middle-ink); --lvl-soft: var(--lvl-middle-soft); }
.lvl-high { --lvl: var(--lvl-high); --lvl-ink: var(--lvl-high-ink); --lvl-soft: var(--lvl-high-soft); }
.lvl-college { --lvl: var(--lvl-college); --lvl-ink: var(--lvl-college-ink); --lvl-soft: var(--lvl-college-soft); }

/* progress */
.bar { height: 6px; border-radius: 999px; background: var(--sunken); overflow: hidden; }
.bar > span { display: block; height: 100%; border-radius: inherit; background: var(--solved); transition: width .3s; }
.ring-track { fill: none; stroke: var(--sunken); stroke-width: 3.5; }
.ring-fill { fill: none; stroke: var(--solved); stroke-width: 3.5; stroke-linecap: round; transition: stroke-dasharray .3s; }

/* difficulty dots */
.dots { display: inline-flex; gap: 3px; align-items: center; }
.dots i { width: 6px; height: 6px; border-radius: 50%; background: var(--line-strong); }
.dots.d1 i:nth-child(-n+1), .dots.d2 i:nth-child(-n+2), .dots.d3 i:nth-child(-n+3) { background: var(--ink-2); }

/* ---------- home ---------- */
.home { padding-top: 40px; }
.home-top {
  display: grid; grid-template-columns: 1fr; gap: 24px; align-items: end;
  padding-bottom: 28px;
}
.home-intro h1 { margin-top: 10px; font-size: clamp(25px, 3vw, 32px); line-height: 1.25; }
.home-stats { min-width: 0; }
.stat-num { display: flex; align-items: baseline; gap: 6px; margin-bottom: 10px; }
.stat-num .mono { font-size: 28px; font-weight: 500; color: var(--ink); letter-spacing: -0.02em; }
.stat-of { color: var(--muted); font-size: 15px; }
@media (min-width: 760px) {
  .home { padding-top: 56px; }
  .home-top { grid-template-columns: 1fr 280px; gap: 48px; }
  .home-stats { padding-bottom: 6px; }
}

.continue {
  display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 16px;
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card);
  padding: 18px 20px; margin-bottom: 28px;
}
.continue-text { min-width: 0; }
.continue-title { font-family: var(--f-head); font-weight: 600; font-size: 19px; line-height: 1.25; margin-top: 4px; }
.continue-meta { color: var(--solved-ink); margin-top: 3px; }
.continue-actions { display: flex; gap: 8px; flex-wrap: wrap; }
@media (max-width: 520px) { .continue-actions, .continue-actions .btn { width: 100%; } .continue-actions .btn { flex: 1; } }

.search { position: relative; margin-bottom: 8px; }
.search .icon { position: absolute; left: 14px; top: 50%; transform: translateY(-50%); color: var(--muted); pointer-events: none; }
.search input {
  width: 100%; height: 48px; border-radius: var(--r-ctl); border: 1px solid var(--line);
  background: var(--surface); color: var(--ink); font: inherit; font-size: 16px; padding: 0 16px 0 44px;
}
.search input::placeholder { color: var(--muted); }
.search input:hover { border-color: var(--line-strong); }
.search input:focus-visible { outline: 2px solid var(--focus-ring); outline-offset: 1px; border-radius: var(--r-ctl); }

.level { margin-top: 36px; }
.level-head { display: flex; align-items: baseline; justify-content: space-between; gap: 12px; margin-bottom: 14px; }
.level-head h2 { display: flex; align-items: center; gap: 10px; font-size: 21px; }
.level-dot { width: 10px; height: 10px; border-radius: 3px; background: var(--lvl); }
.cards { list-style: none; margin: 0; padding: 0; display: grid; grid-template-columns: 1fr; gap: 12px; }
@media (min-width: 620px) { .cards { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
@media (min-width: 960px) { .cards { grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 14px; } }
.card-li { display: flex; min-width: 0; }
.card {
  flex: 1; min-width: 0; display: flex; flex-direction: column;
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card);
  padding: 18px 20px 16px; color: var(--ink); text-decoration: none;
  transition: border-color .15s, transform .15s;
}
a.card:hover { border-color: var(--lvl); }
a.card:focus-visible { border-radius: var(--r-card); }
.card-top { display: flex; align-items: center; justify-content: space-between; gap: 8px; margin-bottom: 10px; min-height: 34px; }
.card-num { font-size: 13px; font-weight: 600; color: var(--lvl-ink); }
.card-ring { display: inline-flex; }
.card.is-done .ring-fill { stroke: var(--solved); }
.card-title { font-size: 17.5px; line-height: 1.25; letter-spacing: -0.01em; }
.card-summary { color: var(--muted); font-size: 14.5px; line-height: 1.5; margin-top: 6px; flex: 1; }
.card-foot { display: flex; justify-content: space-between; gap: 8px; margin-top: 14px; padding-top: 12px; border-top: 1px solid var(--line); color: var(--muted); font-size: 12px; }
.card-solved { color: var(--solved-ink); }
.card.is-soon { background: transparent; border-style: dashed; }
.card.is-soon .card-title { color: var(--muted); }
.card.is-soon .card-num { color: var(--muted); }
.soon-chip {
  font-family: var(--f-mono); font-size: 11px; letter-spacing: .04em; text-transform: uppercase;
  color: var(--muted); border: 1px solid var(--line); border-radius: 999px; padding: 3px 10px;
}
.no-match { margin-top: 32px; color: var(--muted); }

/* ---------- topic ---------- */
.topic { padding-top: 24px; }
.topic-head { max-width: var(--read); }
.back {
  display: inline-flex; align-items: center; gap: 4px; color: var(--muted); text-decoration: none; font-weight: 500;
  font-size: 14.5px; margin-left: -6px; padding: 4px 8px 4px 4px; border-radius: 10px;
}
.back:hover { color: var(--ink); background: var(--sunken); }
.topic-meta { display: flex; align-items: center; flex-wrap: wrap; gap: 8px 12px; margin: 22px 0 14px; }
.topic-head h1 { max-width: 22ch; }
.topic-summary { color: var(--muted); font-size: 17.5px; line-height: 1.55; margin-top: 12px; max-width: 60ch; }

.tabs {
  display: inline-flex; gap: 2px; padding: 4px; margin-top: 26px;
  background: var(--sunken); border-radius: 14px; max-width: 100%;
}
.tabs [role="tab"] {
  font: inherit; font-weight: 600; font-size: 15px; color: var(--muted);
  border: 0; background: transparent; border-radius: 10px; height: 38px; padding: 0 16px; cursor: pointer;
  display: inline-flex; align-items: center; gap: 8px;
}
.tabs [role="tab"]:hover { color: var(--ink); }
.tabs [role="tab"][aria-selected="true"] { background: var(--surface); color: var(--ink); box-shadow: 0 0 0 1px var(--line); }
.tab-count { font-size: 11.5px; color: var(--muted); background: var(--sunken); border-radius: 999px; padding: 1px 7px; }
.tabs [aria-selected="true"] .tab-count { background: var(--lvl-soft); color: var(--lvl-ink); }
.tabpanel { margin-top: 32px; outline: none; }

/* learn */
.learn { display: grid; grid-template-columns: minmax(0, 1fr); gap: 48px; }
.reading { max-width: var(--read); min-width: 0; }
@media (min-width: 1040px) { .learn { grid-template-columns: minmax(0, var(--read)) 220px; justify-content: space-between; } }
.toc { display: none; }
@media (min-width: 1040px) {
  .toc { display: block; position: sticky; top: calc(var(--header-h) + 28px); align-self: start; }
}
.toc ol { list-style: none; margin: 12px 0 18px; padding: 0; border-left: 1px solid var(--line); }
.toc li a {
  display: block; padding: 5px 0 5px 14px; margin-left: -1px; border-left: 2px solid transparent;
  color: var(--muted); text-decoration: none; font-size: 14px; line-height: 1.4;
}
.toc li a:hover { color: var(--ink); }
.toc li a.is-current { color: var(--ink); border-left-color: var(--lvl); font-weight: 500; }
.toc-cta { width: 100%; }

.reading > * + * { margin-top: 18px; }
.reading .th-h { font-size: 24px; margin-top: 44px; scroll-margin-top: calc(var(--header-h) + 20px); outline: none; }
.reading .th-h:first-child { margin-top: 0; }
.reading .th-h + * { margin-top: 12px; }
.th-p, .th-list { font-size: 17px; line-height: 1.7; color: var(--ink); }
.th-list { padding-left: 1.3em; }
.th-list li + li { margin-top: 6px; }
.th-list li::marker { color: var(--muted); }

.th-formula {
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card);
  padding: 6px 22px;
}
.formula-math { overflow-x: auto; overflow-y: hidden; }
.formula-math .katex-display { margin: 14px 0; }
.formula-note { color: var(--muted); font-size: 14.5px; border-top: 1px solid var(--line); padding: 10px 0 12px; }

.th-example {
  border: 1px solid var(--line); border-radius: var(--r-card); background: var(--surface);
  padding: 20px 22px;
}
.th-example h3 { margin-top: 6px; font-size: 19px; }
.ex-problem { margin-top: 10px; font-size: 16.5px; }
.steps { list-style: none; counter-reset: st; margin: 16px 0 0; padding: 0; }
.steps > li {
  counter-increment: st; position: relative; padding: 0 0 0 36px; min-height: 24px;
}
.steps > li + li { margin-top: 12px; }
.steps > li::before {
  content: counter(st); position: absolute; left: 0; top: 1px; width: 24px; height: 24px; border-radius: 8px;
  background: var(--sunken); color: var(--muted); font-family: var(--f-mono); font-size: 12px; font-weight: 600;
  display: flex; align-items: center; justify-content: center;
}
.steps > li > span { display: block; min-width: 0; overflow-x: auto; overflow-y: hidden; }
.ex-result {
  margin-top: 18px; background: var(--sunken); border-radius: var(--r-inner); padding: 12px 16px;
  display: flex; flex-wrap: wrap; align-items: baseline; gap: 6px 14px;
}
.ex-result > div { min-width: 0; max-width: 100%; overflow-x: auto; overflow-y: hidden; }
.rv-label {
  font-family: var(--f-mono); font-size: 11px; font-weight: 600; letter-spacing: .06em; text-transform: uppercase; color: var(--muted);
}

.th-callout { border-radius: var(--r-card); padding: 16px 20px; background: var(--accent-soft); }
.th-callout p:last-child { font-size: 16px; }
.callout-label { display: flex; align-items: center; gap: 7px; font-family: var(--f-mono); font-size: 11.5px; font-weight: 600; letter-spacing: .06em; text-transform: uppercase; margin-bottom: 6px !important; }
.th-callout.is-tip { background: var(--solved-soft); }
.th-callout.is-tip .callout-label { color: var(--solved-ink); }
.th-callout.is-pitfall { background: var(--pitfall-soft); }
.th-callout.is-pitfall .callout-label { color: var(--pitfall-ink); }
.th-callout.is-idea { background: var(--accent-soft); }
.th-callout.is-idea .callout-label { color: var(--accent-ink); }

.table-wrap { overflow-x: auto; border: 1px solid var(--line); border-radius: var(--r-card); background: var(--surface); }
.table-wrap table { border-collapse: collapse; width: 100%; font-size: 15px; font-variant-numeric: tabular-nums; }
.table-wrap th, .table-wrap td { text-align: left; padding: 11px 16px; border-bottom: 1px solid var(--line); vertical-align: top; }
.table-wrap th { font-family: var(--f-mono); font-size: 11.5px; font-weight: 600; letter-spacing: .05em; text-transform: uppercase; color: var(--muted); background: var(--sunken); white-space: nowrap; }
.table-wrap tr:last-child td { border-bottom: 0; }

.learn-end {
  margin-top: 48px !important; display: flex; align-items: center; justify-content: space-between; gap: 16px; flex-wrap: wrap;
  border-top: 1px solid var(--line); padding-top: 22px;
}
.learn-end p { font-family: var(--f-head); font-weight: 600; font-size: 19px; }

/* math */
.katex { font-size: 1.1em; position: relative; }
.katex-display > .katex { width: max-content; min-width: 100%; }
.math-display { display: block; overflow-x: auto; overflow-y: hidden; max-width: 100%; padding: 2px 0; }
.math-display .katex-display { margin: 10px 0; }
.math-wide { display: inline-block; max-width: 100%; overflow-x: auto; overflow-y: hidden; vertical-align: bottom; padding: 2px 0; }
.math-err { font-family: var(--f-mono); font-size: .85em; color: var(--muted); background: var(--sunken); border-radius: 6px; padding: 1px 5px; overflow-wrap: anywhere; }

/* plot */
.plot { margin: 0; background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card); padding: 16px; min-width: 0; }
.reading > .th-plot { margin-top: 24px; margin-bottom: 6px; }
.plot-title { font-family: var(--f-head); font-weight: 600; font-size: 16.5px; line-height: 1.3; padding: 2px 4px 12px; }
.plot-frame { position: relative; border-radius: 12px; overflow: hidden; background: var(--surface); border: 1px solid var(--line); }
.plot-svg { display: block; width: 100%; height: auto; touch-action: pan-y; }
.plot-svg .grid-major { stroke: var(--plot-grid); stroke-width: 1; fill: none; }
.plot-svg .grid-minor { stroke: var(--plot-grid-minor); stroke-width: 1; fill: none; }
.plot-svg .axis { stroke: var(--plot-axis); stroke-width: 1.25; fill: none; }
.plot-svg .tick { stroke: var(--plot-axis); stroke-width: 1; }
.plot-svg .plot-ticks text {
  font-family: var(--f-mono); font-size: 11px; fill: var(--muted);
  paint-order: stroke; stroke: var(--surface); stroke-width: 3px; stroke-linejoin: round;
}
.plot-svg .plot-ticks .axis-name { font-style: italic; font-family: 'KaTeX_Math', var(--f-body); font-size: 15px; fill: var(--ink-2); }
.plot-svg .curve { fill: none; stroke-width: 2.25; stroke-linecap: round; stroke-linejoin: round; }
.plot-svg .curve.is-level { stroke-width: 1.6; }
.plot-svg .pt { stroke: var(--surface); stroke-width: 1.5; }
.plot-svg .trace-guide { stroke: var(--muted); stroke-width: 1; stroke-dasharray: 3 4; fill: none; }
.plot-svg .trace-dot { stroke: var(--surface); stroke-width: 2.5; }
.plot-svg .s1 { stroke: var(--s1); } .plot-svg .pt.s1, .plot-svg .trace-dot.s1 { fill: var(--s1); stroke: var(--surface); }
.plot-svg .s2 { stroke: var(--s2); } .plot-svg .pt.s2, .plot-svg .trace-dot.s2 { fill: var(--s2); stroke: var(--surface); }
.plot-svg .s3 { stroke: var(--s3); } .plot-svg .pt.s3, .plot-svg .trace-dot.s3 { fill: var(--s3); stroke: var(--surface); }
.plot-svg .s4 { stroke: var(--s4); } .plot-svg .pt.s4, .plot-svg .trace-dot.s4 { fill: var(--s4); stroke: var(--surface); }
.plot-svg .s5 { stroke: var(--s5); } .plot-svg .pt.s5, .plot-svg .trace-dot.s5 { fill: var(--s5); stroke: var(--surface); }
.plot-svg .s6 { stroke: var(--s6); } .plot-svg .pt.s6, .plot-svg .trace-dot.s6 { fill: var(--s6); stroke: var(--surface); }
.plot-err {
  position: absolute; left: 8px; right: 8px; bottom: 8px; font-family: var(--f-mono); font-size: 12px; line-height: 1.4;
  color: var(--pitfall-ink); background: var(--pitfall-soft); border-radius: 8px; padding: 6px 10px;
}
.plot-err[hidden] { display: none; }
.plot > .plot-err { position: static; margin-top: 8px; }
.plot-legend { list-style: none; margin: 12px 0 0; padding: 0 4px; display: flex; flex-wrap: wrap; gap: 6px 18px; font-size: 14px; color: var(--ink-2); }
.plot-key { display: inline-flex; align-items: center; gap: 8px; }
.plot-swatch { width: 18px; height: 3px; border-radius: 2px; background: var(--sw); }
.plot-swatch.is-dot { width: 9px; height: 9px; border-radius: 50%; }
.plot-key.s1 { --sw: var(--s1); } .plot-key.s2 { --sw: var(--s2); } .plot-key.s3 { --sw: var(--s3); }
.plot-key.s4 { --sw: var(--s4); } .plot-key.s5 { --sw: var(--s5); } .plot-key.s6 { --sw: var(--s6); }
.plot-controls { margin-top: 14px; display: grid; gap: 4px; background: var(--sunken); border-radius: var(--r-inner); padding: 8px 14px; }
.plot-slider { display: grid; grid-template-columns: minmax(28px, auto) minmax(0, 1fr) auto; align-items: center; gap: 12px; min-height: 36px; }
.plot-slider-label { font-size: 14px; color: var(--ink-2); white-space: nowrap; }
.plot-slider-label.is-symbol { font-family: 'KaTeX_Math', var(--f-body); font-style: italic; font-size: 17px; min-width: 18px; }
.plot-slider-val { font-family: var(--f-mono); font-size: 12.5px; color: var(--ink); text-align: right; min-width: 48px; font-variant-numeric: tabular-nums; }
.plot-slider.is-trace { grid-template-columns: minmax(28px, auto) minmax(0, 1fr); }
.plot-slider.is-trace .plot-slider-val { grid-column: 1 / -1; text-align: left; color: var(--muted); padding-bottom: 4px; }
.plot-slider.is-trace .plot-slider-label::after { content: ' trace'; font-family: var(--f-body); font-style: normal; font-size: 13px; color: var(--muted); }
@media (min-width: 560px) {
  .plot-slider.is-trace { grid-template-columns: minmax(28px, auto) minmax(0, 1fr) minmax(170px, auto); }
  .plot-slider.is-trace .plot-slider-val { grid-column: auto; text-align: right; padding-bottom: 0; }
}
.plot-caption { color: var(--muted); font-size: 14.5px; line-height: 1.55; padding: 12px 4px 2px; }

input[type="range"] {
  -webkit-appearance: none; appearance: none; width: 100%; height: 28px; background: transparent; margin: 0; cursor: pointer;
}
input[type="range"]::-webkit-slider-runnable-track { height: 4px; border-radius: 999px; background: var(--line-strong); }
input[type="range"]::-moz-range-track { height: 4px; border-radius: 999px; background: var(--line-strong); }
input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none; appearance: none; width: 18px; height: 18px; border-radius: 50%; margin-top: -7px;
  background: var(--surface); border: 2px solid var(--accent);
}
input[type="range"]::-moz-range-thumb { width: 14px; height: 14px; border-radius: 50%; background: var(--surface); border: 2px solid var(--accent); }
input[type="range"]:focus-visible { outline: none; }
input[type="range"]:focus-visible::-webkit-slider-thumb { box-shadow: 0 0 0 3px var(--surface), 0 0 0 5px var(--focus-ring); }
input[type="range"]:focus-visible::-moz-range-thumb { box-shadow: 0 0 0 3px var(--surface), 0 0 0 5px var(--focus-ring); }

/* practice */
.practice { max-width: 860px; }
.pr-progress { display: flex; align-items: center; gap: 14px; margin-bottom: 16px; }
.pr-progress .mono { white-space: nowrap; color: var(--ink-2); }
.pr-progress .bar { flex: 1; max-width: 320px; }
.toolbar { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin-bottom: 20px; }
.toolbar-gap { flex: 1; }
.seg { display: inline-flex; gap: 2px; padding: 3px; background: var(--sunken); border-radius: 12px; max-width: 100%; overflow-x: auto; }
.seg button {
  font: inherit; font-size: 14px; font-weight: 500; color: var(--muted); background: transparent; border: 0; border-radius: 9px;
  height: 32px; padding: 0 11px; cursor: pointer; display: inline-flex; align-items: center; gap: 7px; white-space: nowrap;
}
.seg button:hover { color: var(--ink); }
.seg button[aria-pressed="true"] { background: var(--surface); color: var(--ink); box-shadow: 0 0 0 1px var(--line); }
.btn-toggle { background: transparent; }
.switch { width: 28px; height: 16px; border-radius: 999px; background: var(--line-strong); position: relative; transition: background-color .15s; }
.switch::after { content: ''; position: absolute; top: 2px; left: 2px; width: 12px; height: 12px; border-radius: 50%; background: var(--surface); transition: transform .15s; }
.btn-toggle[aria-pressed="true"] .switch { background: var(--accent); }
.btn-toggle[aria-pressed="true"] .switch::after { transform: translateX(12px); }
@media (max-width: 640px) {
  .toolbar-gap { display: none; }
  .seg { width: 100%; }
  .seg button { flex: 1; justify-content: center; padding: 0 6px; }
  .seg button .dots { display: none; }
}

.qlist { list-style: none; margin: 0; padding: 0; display: grid; gap: 14px; }
.qcard {
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card);
  padding: 18px 20px 18px; min-width: 0; transition: border-color .2s;
}
.qcard.is-solved { border-color: color-mix(in srgb, var(--solved) 45%, var(--line)); }
.q-head { display: flex; align-items: center; flex-wrap: wrap; gap: 6px 12px; }
.q-num {
  font-weight: 600; font-size: 12px; color: var(--lvl-ink, var(--accent-ink)); background: var(--lvl-soft, var(--accent-soft));
  border-radius: 999px; padding: 2px 9px;
}
.qcard.is-solved .q-num { color: var(--solved-ink); background: var(--solved-soft); }
.q-variant { font-weight: 600; font-size: 14.5px; color: var(--ink-2); }
.q-topic { color: var(--muted); font-size: 12px; flex-basis: 100%; }
.q-body { font-size: 17px; line-height: 1.65; margin-top: 12px; min-width: 0; overflow-wrap: break-word; }
.q-actions { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 16px; }
.btn-reveal { background: var(--surface); }
.btn-reveal[aria-expanded="true"] { background: var(--sunken); border-color: var(--sunken); }
.btn-got { margin-left: auto; background: transparent; color: var(--muted); }
.btn-got .icon { color: var(--line-strong); }
.btn-got:hover .icon { color: var(--solved); }
.btn-got[aria-pressed="true"] { background: var(--solved-soft); border-color: transparent; color: var(--solved-ink); }
.btn-got[aria-pressed="true"] .icon { color: var(--solved-ink); }
@media (max-width: 420px) { .btn-got { margin-left: 0; } }
.q-reveal { margin-top: 14px; border-radius: var(--r-inner); padding: 12px 16px; background: var(--sunken); min-width: 0; }
.q-reveal[hidden] { display: none; }
.q-reveal > div, .q-reveal .steps > li > span { overflow-x: auto; overflow-y: hidden; }
.q-reveal .rv-label { display: block; margin-bottom: 4px; }
.rv-hint { background: var(--warn-soft); }
.rv-hint .rv-label { color: var(--warn-ink); }
.rv-answer { background: var(--accent-soft); }
.rv-answer .rv-label { color: var(--accent-ink); }
.rv-answer > div { font-size: 16.5px; }
.rv-steps { background: transparent; border: 1px solid var(--line); }
.rv-steps .steps { margin-top: 10px; }
.rv-steps .steps > li::before { background: var(--sunken); }
.empty-inline { border: 1px dashed var(--line-strong); border-radius: var(--r-card); padding: 28px; color: var(--muted); text-align: center; }

/* sources */
.sources { max-width: 860px; }
.sources-intro { margin-bottom: 16px; }
.source-grid { list-style: none; margin: 0; padding: 0; display: grid; gap: 12px; grid-template-columns: 1fr; }
@media (min-width: 640px) { .source-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
.source-grid li { display: flex; min-width: 0; }
.source {
  flex: 1; min-width: 0; display: flex; flex-direction: column; gap: 6px; padding: 18px 20px;
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card); text-decoration: none; color: var(--ink);
}
.source:hover { border-color: var(--line-strong); }
.source:hover .source-title { color: var(--accent-ink); }
.source:focus-visible { border-radius: var(--r-card); }
.kind { align-self: flex-start; font-size: 11px; font-weight: 600; letter-spacing: .05em; text-transform: uppercase; border-radius: 999px; padding: 3px 10px; background: var(--sunken); color: var(--muted); }
.kind-course { background: var(--lvl-high-soft); color: var(--lvl-high-ink); }
.kind-book { background: var(--lvl-middle-soft); color: var(--lvl-middle-ink); }
.kind-video { background: var(--lvl-college-soft); color: var(--lvl-college-ink); }
.kind-reference { background: var(--sunken); color: var(--ink-2); }
.source-title { font-family: var(--f-head); font-weight: 600; font-size: 17px; line-height: 1.3; margin-top: 4px; }
.source-by { color: var(--muted); font-size: 14.5px; }
.source-host { margin-top: auto; padding-top: 8px; color: var(--muted); display: inline-flex; align-items: center; gap: 6px; overflow-wrap: anywhere; }

/* pager */
.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; margin-top: 64px; max-width: var(--read); }
.pager-link {
  display: flex; flex-direction: column; gap: 4px; padding: 14px 18px; min-width: 0;
  border: 1px solid var(--line); border-radius: var(--r-card); background: var(--surface); text-decoration: none; color: var(--ink);
}
.pager-link:hover { border-color: var(--line-strong); }
.pager-link.next { text-align: right; align-items: flex-end; grid-column: 2; }
.pager-dir { display: inline-flex; align-items: center; gap: 4px; font-family: var(--f-mono); font-size: 11.5px; text-transform: uppercase; letter-spacing: .05em; color: var(--muted); }
.pager-title { font-family: var(--f-head); font-weight: 600; font-size: 15.5px; line-height: 1.3; }
@media (max-width: 520px) { .pager { grid-template-columns: 1fr; } .pager-link.next { grid-column: 1; } }

/* empty / coming soon */
.empty { max-width: 560px; padding: 64px 0 24px; display: flex; flex-direction: column; align-items: flex-start; gap: 14px; }
.empty p { color: var(--muted); font-size: 17px; }

/* ---------- drill ---------- */
.drill { padding-top: 40px; }
.drill-head { max-width: var(--read); }
.drill-head h1 { margin-top: 10px; }
.drill-panel { margin-top: 32px; max-width: var(--read); background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card); padding: 20px; }
.level-pick { border: 0; margin: 0; padding: 0; display: grid; gap: 10px; min-width: 0; }
.level-pick legend { padding: 0; margin-bottom: 10px; }
.pick {
  font: inherit; color: var(--ink); text-align: left; cursor: pointer;
  display: flex; align-items: center; gap: 14px; padding: 14px 16px; border-radius: 16px;
  border: 1px solid var(--line); background: var(--surface);
}
.pick:hover { border-color: var(--line-strong); }
.pick-check {
  width: 24px; height: 24px; border-radius: 8px; border: 1.5px solid var(--line-strong); display: inline-flex; align-items: center; justify-content: center; color: transparent; flex: none;
}
.pick[aria-pressed="true"] { border-color: var(--lvl); background: var(--lvl-soft); }
.pick[aria-pressed="true"] .pick-check { background: var(--lvl); border-color: var(--lvl); color: var(--surface); }
.pick-text { display: flex; flex-direction: column; min-width: 0; }
.pick-name { font-weight: 600; font-size: 16px; }
.pick-meta { color: var(--muted); font-size: 12px; }
.drill-go { display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap; margin-top: 18px; padding-top: 18px; border-top: 1px solid var(--line); }

/* ---------- focus mode ---------- */
.focus {
  position: fixed; inset: 0; z-index: 50; background: var(--page); overflow-y: auto; overscroll-behavior: contain;
  display: flex; flex-direction: column;
}
.focus-bar { position: sticky; top: 0; z-index: 2; background: var(--page); border-bottom: 1px solid var(--line); }
.focus-bar-in { display: flex; align-items: center; gap: 14px; height: var(--header-h); max-width: calc(780px + 2 * var(--gutter)); }
.focus-ctx { display: flex; flex-direction: column; min-width: 0; flex: 1; line-height: 1.3; }
.focus-title { font-family: var(--f-head); font-weight: 600; font-size: 16px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.focus-count { color: var(--ink-2); font-size: 13px; }
.focus-track { display: flex; gap: 3px; max-width: 780px; margin: 0 auto; padding: 0 var(--gutter) 10px; }
.focus-track span { flex: 1; height: 4px; border-radius: 999px; background: var(--line); }
.focus-track span.is-solved { background: var(--solved); }
.focus-track span.is-current { background: var(--ink-2); }
.focus-track span.is-solved.is-current { background: var(--solved-ink); }
.focus-body { flex: 1; max-width: calc(780px + 2 * var(--gutter)); padding-top: 32px; padding-bottom: 32px; }
.focus .qcard { padding: 26px 28px; box-shadow: var(--shadow); }
.focus .q-body { font-size: 19px; margin-top: 16px; }
.focus .q-topic { flex-basis: auto; margin-left: auto; }
@media (max-width: 560px) { .focus .qcard { padding: 20px 18px; } .focus .q-body { font-size: 17.5px; } .focus .q-topic { flex-basis: 100%; margin-left: 0; } }
.focus-summary { background: var(--surface); border: 1px solid var(--line); border-radius: var(--r-card); padding: 32px 28px; box-shadow: var(--shadow); }
.focus-summary h2 { font-size: 28px; margin-top: 10px; }
.focus-summary .muted { margin-top: 8px; }
.focus-summary-actions { display: flex; gap: 10px; flex-wrap: wrap; margin-top: 24px; }
.focus-foot { position: sticky; bottom: 0; background: var(--page); border-top: 1px solid var(--line); }
.focus-foot-in { display: flex; align-items: center; justify-content: space-between; gap: 12px; height: 72px; max-width: calc(780px + 2 * var(--gutter)); }
.keys { color: var(--muted); font-size: 13px; display: flex; align-items: center; flex-wrap: wrap; justify-content: center; gap: 4px; }
kbd {
  font-family: var(--f-mono); font-size: 11.5px; min-width: 22px; height: 22px; padding: 0 5px; border-radius: 6px;
  display: inline-flex; align-items: center; justify-content: center; background: var(--surface); border: 1px solid var(--line); color: var(--ink-2);
  margin-left: 6px;
}
kbd + kbd { margin-left: 2px; }
.keys kbd:first-child { margin-left: 0; }
@media (max-width: 720px) { .keys { display: none; } }

/* ---------- study dashboard ---------- */
.has-dashboard { height: 100vh; height: 100dvh; min-height: 0; overflow: hidden; }
.has-dashboard main { min-height: 0; }
.has-dashboard .site-footer { display: none; }
.has-dashboard .header-in { max-width: none; padding: 0 24px; }
.dashboard { display: grid; grid-template-columns: 300px minmax(0, 1fr); height: calc(100vh - var(--header-h)); height: calc(100dvh - var(--header-h)); }
.topic-sidebar { display: flex; flex-direction: column; min-height: 0; min-width: 0; border-right: 1px solid var(--line); background: var(--surface); }
.sidebar-head { display: flex; align-items: center; justify-content: space-between; padding: 26px 20px 18px; }
.sidebar-head h2 { display: flex; align-items: baseline; gap: 10px; font-size: 22px; margin-top: 5px; }
.sidebar-head .overline { font-size: 10px; }
.sidebar-search { margin: 0 16px 16px; }
.sidebar-search input { height: 40px; font-size: 14px; background: var(--page); }
.sidebar-scroll { flex: 1; min-height: 0; overflow-y: auto; padding: 0 12px 20px; scrollbar-width: thin; scrollbar-color: var(--line-strong) transparent; overscroll-behavior: contain; }
.sidebar-group + .sidebar-group { margin-top: 24px; }
.sidebar-group h3 { display: flex; align-items: center; gap: 8px; padding: 10px 10px 12px; font: 500 10px var(--f-mono); text-transform: uppercase; letter-spacing: .07em; color: var(--muted); }
.sidebar-group .level-dot { width: 7px; height: 7px; border-radius: 2px; flex: none; }
.sidebar-group ul { padding: 0; margin: 0; list-style: none; }
.sidebar-topic { display: grid; grid-template-columns: 20px minmax(0, 1fr) auto; align-items: start; gap: 9px; padding: 12px 10px; margin: 2px 0; border: 1px solid transparent; border-radius: 12px; color: var(--ink-2); text-decoration: none; }
.sidebar-topic:hover { background: var(--page); color: var(--ink); }
.sidebar-topic.is-active { background: var(--accent-soft); border-color: var(--accent-soft); color: var(--accent-ink); }
.sidebar-num { color: var(--muted); font-size: 11px; line-height: 20px; }
.sidebar-topic.is-active .sidebar-num { color: var(--accent-ink); }
.sidebar-topic-title { font-size: 13.5px; font-weight: 500; line-height: 1.45; }
.sidebar-topic.is-active .sidebar-topic-title { font-weight: 600; }
.sidebar-topic-count { font-size: 10px; line-height: 20px; color: var(--muted); }
.sidebar-topic.is-soon { color: var(--muted); }
.sidebar-topic.is-soon > .mono:last-child { font-size: 9px; padding-top: 3px; }
.sidebar-scroll .no-match { padding: 0 10px; font-size: 14px; }
.sidebar-foot { border-top: 1px solid var(--line); padding: 18px 22px; }
.sidebar-progress-label { display: flex; align-items: center; justify-content: space-between; margin-bottom: 10px; gap: 8px; }
.sidebar-progress-label .overline { font-size: 10px; }
.sidebar-progress-label .mono { font-size: 11px; }
.sidebar-foot p { color: var(--muted); font-size: 12px; margin-top: 8px; }
.workspace { min-width: 0; min-height: 0; overflow-y: auto; overscroll-behavior: contain; scrollbar-width: thin; }
.workspace-inner { max-width: 1000px; margin: 0 auto; padding: 32px 40px 72px; }
.workspace-head { padding-bottom: 26px; margin-bottom: 20px; border-bottom: 1px solid var(--line); }
.workspace-breadcrumb { display: flex; align-items: center; gap: 12px; margin-bottom: 16px; }
.workspace-breadcrumb .chip { font-size: 10px; height: 24px; }
.workspace-breadcrumb .mono { font-size: 11px; }
.workspace-head h1 { font-size: clamp(26px, 2.8vw, 34px); max-width: 30ch; line-height: 1.15; }
.workspace-head h1:focus { outline: none; }
.workspace-summary { margin-top: 10px; color: var(--muted); font-size: 15px; line-height: 1.6; max-width: 70ch; }
.study-tools, .study-tools-left, .study-tools-right { display: flex; align-items: center; gap: 8px; flex-wrap: wrap; }
.study-tools { justify-content: space-between; gap: 12px; }
.study-tools .btn { font-size: 12px; height: 34px; padding: 0 10px; }
.study-tools .btn-toggle { padding-left: 0; border-color: transparent; }
.study-tools .btn-toggle .switch { width: 25px; height: 16px; }
.study-tools .btn-toggle[aria-pressed="true"] .switch::after { transform: translateX(9px); }
#difficulty-filter { height: 34px; max-width: 100%; border-radius: 10px; border: 1px solid var(--line); background: var(--surface); color: var(--ink-2); font: 500 12px var(--f-body); padding: 0 26px 0 10px; cursor: pointer; }
.question-set-head { display: flex; justify-content: space-between; align-items: center; margin: 26px 0 16px; gap: 12px; }
.question-set-head .overline { color: var(--ink); font-size: 12px; margin-bottom: 2px; }
.question-set-head > div > .mono { font-size: 11px; }
.question-paging { display: flex; align-items: center; gap: 8px; }
.question-paging > .mono { font-size: 11px; min-width: 55px; text-align: center; }
.question-paging .btn { width: 30px; height: 30px; border: 1px solid var(--line); background: var(--surface); }
.question-numbers { display: flex; gap: 7px; overflow-x: auto; min-width: 0; padding: 3px 3px 10px; margin: -3px -3px 6px; scrollbar-width: thin; }
.question-number { position: relative; flex: 0 0 34px; height: 34px; padding: 0; font-size: 11px; border-radius: 10px; border: 1px solid var(--line); background: var(--surface); color: var(--muted); cursor: pointer; }
.question-number:hover { color: var(--ink); border-color: var(--line-strong); }
.question-number.is-solved { color: var(--solved-ink); background: var(--solved-soft); border-color: var(--solved-soft); }
.question-number[aria-current] { color: var(--on-accent); background: var(--accent); border-color: var(--accent); }
.question-number.is-solved::after { content: ''; position: absolute; width: 4px; height: 4px; border-radius: 50%; background: currentColor; bottom: 3px; left: calc(50% - 2px); }
.study-stage > .qcard { margin-top: 6px; padding: 24px 26px 20px; }
.study-stage > .qcard .q-body { font-size: 20px; line-height: 1.7; min-height: 92px; margin-top: 24px; padding-bottom: 16px; }
.study-stage > .qcard .q-head { gap: 8px 12px; }
.study-stage > .qcard .q-variant { font-size: 13px; }
.study-stage > .qcard .dots { margin-left: auto; }
.study-stage > .qcard .q-actions { border-top: 1px solid var(--line); padding-top: 16px; margin-top: 0; }
.study-stage > .qcard .btn-got { margin-left: auto; }
.dashboard-tabs { display: flex; background: transparent; border-radius: 0; padding: 0; gap: 26px; margin-top: 26px; border-bottom: 1px solid var(--line); }
.dashboard-tabs [role="tab"] { position: relative; padding: 0 0 14px; height: 42px; border-radius: 0; font-size: 14px; }
.dashboard-tabs [role="tab"][aria-selected="true"] { box-shadow: none; color: var(--accent-ink); background: transparent; }
.dashboard-tabs [role="tab"][aria-selected="true"]::after { position: absolute; content: ''; bottom: -1px; left: 0; right: 0; height: 3px; border-radius: 3px 3px 0 0; background: var(--accent); }
.dashboard-tabs [role="tab"]:disabled { opacity: .5; cursor: default; }
.dashboard-panel { margin-top: 28px; }
.dashboard-panel .learn { display: block; }
.dashboard-panel .reading { max-width: none; }
.dashboard-panel .reading .th-h { font-size: 22px; }
.dashboard-panel .th-p, .dashboard-panel .th-list { font-size: 16px; }
.dashboard-panel .sources-intro { font-size: 14px; }
.dashboard-panel .source-grid { margin-top: 18px; }
.answer-final { background: var(--accent-soft); border-radius: 20px; padding: 20px 24px; }
.answer-final .overline { color: var(--accent-ink); margin-bottom: 12px; }
.answer-final > div { font-size: 20px; overflow-x: auto; overflow-y: hidden; }
.answer-working { margin-top: 22px; }
.answer-working .steps { font-size: 16px; margin-top: 24px; }
.workspace .drill { padding: 0; }
.sidebar-close, .sidebar-backdrop, #topic-toggle { display: none; }
@media (min-width: 1600px) { .dashboard { grid-template-columns: 320px minmax(0, 1fr); } .workspace-inner { padding-top: 44px; } }
@media (max-width: 1100px) and (min-width: 801px) { .dashboard { grid-template-columns: 270px minmax(0, 1fr); } .workspace-inner { padding-left: 26px; padding-right: 26px; } .study-tools { gap: 8px; } }
@media (max-width: 800px) {
  .dashboard { grid-template-columns: minmax(0, 1fr); }
  .has-dashboard .header-in { padding: 0 16px; }
  .site-nav [data-nav="topics"] { display: none; }
  #topic-toggle { display: inline-flex; border-color: var(--line); height: 32px; font-size: 13px; }
  .site-nav { gap: 2px; }
  .site-nav a { padding-left: 10px; padding-right: 10px; font-size: 14px; }
  .topic-sidebar { display: none; position: fixed; inset: 0 auto 0 0; z-index: 60; width: min(320px, calc(100vw - 36px)); }
  .sidebar-open .topic-sidebar { display: flex; }
  .sidebar-close { display: inline-flex; }
  .sidebar-open .sidebar-backdrop { display: block; position: fixed; inset: 0; z-index: 55; border: 0; background: rgba(0, 0, 0, .42); cursor: pointer; }
  .workspace-inner { padding: 24px 20px 52px; }
  .workspace-head { padding-bottom: 20px; }
  .workspace-head h1 { font-size: 28px; }
}
@media (max-width: 480px) {
  .workspace-inner { padding-left: 16px; padding-right: 16px; }
  .workspace-head h1 { font-size: 26px; }
  .workspace-summary { font-size: 14px; }
  .study-tools-left, .study-tools-right { justify-content: space-between; width: 100%; }
  .study-tools-right { justify-content: flex-end; gap: 6px; }
  .study-tools-right [data-tool="shuffle"] { margin-right: auto; }
  .study-tools .btn { font-size: 12px; }
  .question-set-head { margin-top: 20px; }
  .question-number { flex-basis: 32px; height: 32px; }
  .question-numbers { gap: 6px; }
  .study-stage > .qcard { padding: 20px 18px 16px; }
  .study-stage > .qcard .q-body { font-size: 18px; margin-top: 20px; }
  .study-stage > .qcard .q-variant { flex: 1; }
  .dashboard-tabs { gap: 26px; }
  .dashboard-tabs [role="tab"] { font-size: 13px; }
  .dashboard-panel .th-formula { padding-left: 14px; padding-right: 14px; }
  .dashboard-panel .th-example { padding: 18px 16px; }
  .answer-final { padding: 18px; }
}

/* ---------- motion ---------- */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: 0.01ms !important; animation-duration: 0.01ms !important; scroll-behavior: auto !important; }
}

@media print {
  .site-header, .site-footer, .tabs, .pager, .toolbar, .q-actions { display: none !important; }
  body { background: #fff; }
}