@import url('https://fonts.googleapis.com/css2?family=Newsreader:ital,opsz,wght@0,6..72,400;0,6..72,500;0,6..72,600;1,6..72,400;1,6..72,500&family=Space+Grotesk:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500&display=swap');

/* style.css — "Observatory Atelier" theme.
 * Warm scholarly: vellum paper, warm ink, indigo structure + amber spark.
 * Signature: a faint coordinate-graph texture in the math/diagram zones.
 * Light theme on :root, dark on [data-theme="dark"].
 * --sim-* variables are read by physics-sims.js for canvas colors.
 */

/* ----------------------------------------------------------------- tokens */
:root {
  /* paper + ink */
  --paper:      #f6f1e7;   /* warm vellum */
  --paper-elev: #fdfaf1;   /* lighter warm panel (cards, callouts, sim bg) */
  --paper-sunk: #efe8d8;   /* sunk/recessed surfaces */
  --ink:        #1f1d24;   /* warm near-black */
  --ink-soft:   #514c5a;   /* warm muted secondary */
  --ink-mute:   #8c8498;   /* muted captions */
  --rule:       #e3dcc9;   /* warm hairline */
  --border:     #d9d0bb;
  --grid:       #e9e1cd;   /* graph-grid line (math zones) */
  --grid-faint: rgba(120, 105, 70, 0.05); /* body-level ultra-faint grid */

  /* accents */
  --indigo:      #3a3585;   /* structure, links, key-terms */
  --indigo-soft: #e9e5f4;   /* indigo tint */
  --indigo-deep: #2c2868;
  --amber:       #b8772a;   /* the "spark": answers, ✓ progress, accents */
  --amber-soft:  #f4e8d2;
  --amber-deep:  #8f5a1d;
  --rose:        #9a3b32;   /* pitfalls / warnings */
  --rose-soft:   #f4e3df;
  --sage:        #5c6b3e;   /* notes / tips */
  --sage-soft:   #eef0e2;
  --link:        var(--indigo);

  /* simulation canvas palette (read by physics-sims.js) */
  --sim-primary: #3a3585;
  --sim-accent:  #b8772a;
  --sim-accent2: #5c6b3e;
  --sim-grid:    #e6dec9;
  --sim-trace:   #6d4ba8;
  --sim-axis:    #6b6378;
  --sim-bg:      #fdfaf1;
  --sim-text:    #1f1d24;

  /* type */
  --font-display: 'Space Grotesk', system-ui, -apple-system, 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', Roboto, sans-serif;
  --font-body:    'Newsreader', Georgia, 'Source Han Serif SC', 'Noto Serif SC', 'Songti SC', 'STSong', 'SimSun', serif;
  --font-mono:    'JetBrains Mono', 'SF Mono', 'JetBrains Mono', Menlo, Consolas, monospace;

  --sidebar-w: 290px;
  --content-max: 1040px;
  --radius: 7px;
  --radius-sm: 4px;

  /* Backward-compat aliases — older page-scoped <style> blocks and legacy
     markup still reference these pre-redesign token names. They resolve to
     the new tokens, so they follow light/dark automatically (custom
     properties are resolved lazily against the cascade). */
  --primary:      var(--indigo);
  --primary-soft: var(--indigo-soft);
  --heading:      var(--ink);
  --text-soft:    var(--ink-soft);
  --text-mute:    var(--ink-mute);
  --bg-elev:      var(--paper-elev);
  --code-bg:      var(--paper-sunk);
  --sidebar-bg:   var(--paper-sunk);
  --font-head:    var(--font-display);
}

[data-theme="dark"] {
  --paper:      #17151e;   /* deep warm-indigo charcoal */
  --paper-elev: #1f1c28;
  --paper-sunk: #131119;
  --ink:        #e9e4d6;   /* warm starlight off-white */
  --ink-soft:   #b6afbe;
  --ink-mute:   #80798d;
  --rule:       #2d2937;
  --border:     #383343;
  --grid:       #2a2636;
  --grid-faint: rgba(200, 190, 160, 0.04);

  --indigo:      #9d96e6;
  --indigo-soft: #2a2540;
  --indigo-deep: #b8b2ee;
  --amber:       #dca35a;
  --amber-soft:  #3a2c18;
  --amber-deep:  #e9b878;
  --rose:        #e08579;
  --rose-soft:   #3a201d;
  --sage:        #9fb377;
  --sage-soft:   #232a1c;
  --link:        var(--indigo);

  --sim-primary: #9d96e6;
  --sim-accent:  #dca35a;
  --sim-accent2: #9fb377;
  --sim-grid:    #2c2838;
  --sim-trace:   #c4b0ee;
  --sim-axis:    #8a8398;
  --sim-bg:      #1f1c28;
  --sim-text:    #e9e4d6;
}

/* reusable graph-paper texture */
%grid, .has-grid {
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 26px 26px;
}

/* ------------------------------------------------------------------ base */
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font-body);
  font-size: 18px;
  line-height: 1.72;
  background-color: var(--paper);
  /* ultra-faint body grid (signature, but quiet for reading) */
  background-image:
    linear-gradient(var(--grid-faint) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid-faint) 1px, transparent 1px);
  background-size: 28px 28px;
  color: var(--ink);
  font-feature-settings: "kern", "liga", "calt";
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}
::selection { background: var(--amber-soft); color: var(--ink); }

h1, h2, h3, h4 {
  font-family: var(--font-display);
  color: var(--ink);
  line-height: 1.25;
  font-weight: 600;
  letter-spacing: -0.012em;
}
h2 {
  font-size: 1.5rem;
  font-weight: 600;
  margin: 2.4em 0 0.7em;
  padding-bottom: 0.35em;
  border-bottom: 1px solid var(--rule);
  display: flex; align-items: baseline; gap: 0.5em;
}
h2::before { /* small indigo tick — a structural marker, not decoration */
  content: ""; display: inline-block; width: 7px; height: 7px;
  background: var(--indigo); transform: translateY(-2px); flex: 0 0 7px;
}
h3 { font-size: 1.16rem; margin: 1.7em 0 0.5em; color: var(--ink-soft); font-weight: 500; }
h4 { font-size: 1.02rem; margin: 1.2em 0 0.4em; color: var(--ink); font-weight: 600; }
p  { margin: 0.8em 0; }
strong { color: var(--ink); font-weight: 600; }
em { font-style: italic; }
a { color: var(--link); text-decoration: underline; text-decoration-color: color-mix(in srgb, var(--link) 40%, transparent); text-underline-offset: 3px; text-decoration-thickness: 1px; transition: color .15s, text-decoration-color .15s; }
a:hover { color: var(--amber-deep); text-decoration-color: var(--amber); }
code { font-family: var(--font-mono); background: var(--paper-sunk); padding: 0.08em 0.4em; border-radius: var(--radius-sm); font-size: 0.88em; color: var(--indigo-deep); }
[data-theme="dark"] code { color: var(--indigo); }
pre { background: var(--paper-sunk); padding: 1em; border-radius: var(--radius); overflow-x: auto; border: 1px solid var(--rule); }
pre code { background: none; padding: 0; color: var(--ink); }
mjx-container { font-size: 1.04em !important; }
mjx-container[display="true"] { font-size: 1.16em !important; max-width: 100%; overflow-x: auto; overflow-y: hidden; }
.formula { -webkit-overflow-scrolling: touch; }

ul, ol { padding-left: 1.3em; }
li { margin: 0.3em 0; }

/* ---------------------------------------------------------------- layout */
.sidebar {
  position: fixed; top: 0; left: 0; bottom: 0;
  width: var(--sidebar-w);
  background: var(--paper-sunk);
  border-right: 1px solid var(--border);
  overflow-y: auto;
  padding: 20px 14px 40px;
  z-index: 40;
}
.content {
  margin-left: var(--sidebar-w);
  max-width: var(--content-max);
  padding: 52px 54px 130px;
}
.sidebar-toggle {
  display: none; position: fixed; top: 12px; left: 12px; z-index: 50;
  font-size: 1.2rem; font-family: var(--font-display);
  background: var(--paper-elev); color: var(--ink); border: 1px solid var(--border);
  border-radius: var(--radius); width: 42px; height: 42px; cursor: pointer;
}

/* ------------------------------------------------------------- sidebar */
.sidebar-brand { font-family: var(--font-display); font-size: 1.12rem; font-weight: 700; margin-bottom: 14px; letter-spacing: -0.01em; }
.sidebar-brand .home-link { color: var(--ink); text-decoration: none; }
.dark-toggle {
  display: block; width: 100%; margin-bottom: 16px; padding: 8px 10px;
  font-family: var(--font-display); font-size: 0.82rem; font-weight: 500; cursor: pointer;
  background: var(--paper-elev); color: var(--ink-soft); border: 1px solid var(--border); border-radius: var(--radius-sm);
  transition: background .15s, color .15s;
}
.dark-toggle:hover { background: var(--amber-soft); color: var(--amber-deep); border-color: var(--amber); }

/* full-text search (in sidebar) */
.search { margin-bottom: 16px; }
.search input {
  width: 100%; box-sizing: border-box; padding: 8px 11px;
  font-family: var(--font-body); font-size: 0.9rem;
  background: var(--paper-elev); color: var(--ink);
  border: 1px solid var(--border); border-radius: var(--radius-sm);
  transition: border-color .15s, box-shadow .15s;
}
.search input:focus {
  outline: none; border-color: var(--indigo);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--indigo) 18%, transparent);
}
.search input::placeholder { color: var(--ink-mute); }
.search-results { margin-top: 8px; max-height: 56vh; overflow-y: auto; }
.search-results .sr-empty { padding: 6px 2px; color: var(--ink-mute); font-size: 0.82rem; }
.search-results .sr-loading { padding: 6px 2px; color: var(--ink-mute); font-size: 0.82rem; }
.search-results .sr-item {
  display: block; padding: 7px 9px; text-decoration: none; color: var(--ink);
  border-left: 2px solid transparent; border-radius: var(--radius-sm);
}
.search-results .sr-item + .sr-item { margin-top: 1px; }
.search-results .sr-item:hover { background: var(--paper-sunk); border-left-color: var(--amber); }
.search-results .sr-title { display: block; font-family: var(--font-display); font-weight: 600; font-size: 0.82rem; color: var(--indigo-deep); }
[data-theme="dark"] .search-results .sr-title { color: var(--indigo); }
.search-results .sr-snip { display: block; font-size: 0.77rem; color: var(--ink-soft); margin-top: 3px; line-height: 1.4; }
.search-results mark { background: var(--amber-soft); color: var(--amber-deep); padding: 0 1px; border-radius: 2px; }
[data-theme="dark"] .search-results mark { background: color-mix(in srgb, var(--amber) 40%, transparent); color: var(--paper-elev); }
.sem-group { margin-bottom: 4px; }
.sem-group > .sem-summary {
  list-style: none; cursor: pointer; user-select: none;
  font-family: var(--font-display); font-size: 0.72rem; font-weight: 700;
  letter-spacing: 0.05em; text-transform: uppercase; color: var(--ink);
  padding: 9px 11px; margin: 14px 0 5px;
  border-radius: var(--radius-sm);
  background: var(--paper-elev); border: 1px solid var(--border);
  display: flex; align-items: center; gap: 8px;
}
.sem-group > .sem-summary::-webkit-details-marker { display: none; }
.sem-group > .sem-summary::before {
  content: '▸'; color: var(--indigo); font-size: 0.68rem; transition: transform .15s; display: inline-block;
}
.sem-group[open] > .sem-summary::before { transform: rotate(90deg); }
.sidebar-part {
  font-family: var(--font-display); font-size: 0.68rem; font-weight: 600;
  letter-spacing: 0.04em; text-transform: uppercase; color: var(--ink-mute);
  margin: 13px 6px 5px;
}
.sidebar-link {
  display: flex; align-items: baseline; gap: 9px;
  padding: 5px 9px; border-radius: var(--radius-sm);
  color: var(--ink-soft); font-size: 0.9rem; text-decoration: none;
  border-left: 2px solid transparent;
  transition: background .14s, color .14s, border-color .14s;
}
.sidebar-link:hover { background: var(--paper-elev); color: var(--ink); text-decoration: none; border-left-color: var(--rule); }
.sidebar-link.active { background: var(--indigo-soft); color: var(--indigo-deep); font-weight: 600; border-left-color: var(--amber); }
[data-theme="dark"] .sidebar-link.active { color: var(--indigo); }
.sidebar-link.is-ref { opacity: 0.8; }
.sidebar-link.is-ref .sl-title::before { content: '★ '; color: var(--amber); }
.sl-title { display: flex; flex-direction: column; line-height: 1.22; }
.sl-en { font-size: 0.72rem; color: var(--ink-mute); font-family: var(--font-display); font-weight: 400; }
.sidebar-foot { margin-top: 28px; font-size: 0.7rem; color: var(--ink-mute); text-align: center; font-family: var(--font-display); letter-spacing: 0.04em; }

/* -------------------------------------------------------- chapter head */
.chapter-header { margin-bottom: 1.8em; padding-bottom: 1.2em; border-bottom: 1px solid var(--rule); }
.chapter-number {
  font-family: var(--font-display); font-size: 0.76rem; font-weight: 600;
  letter-spacing: 0.08em; text-transform: uppercase; color: var(--amber-deep);
  margin: 0 0 0.5em; display: inline-flex; align-items: center; gap: 0.6em;
}
.chapter-number::after { content: ""; display: inline-block; width: 28px; height: 1px; background: var(--amber); }
.chapter-header h1 { font-size: 2.35rem; margin: 0; font-weight: 600; letter-spacing: -0.02em; }
.en-title { display: block; font-family: var(--font-body); font-style: italic; font-size: 1.08rem; font-weight: 400; color: var(--ink-soft); margin-top: 0.25em; letter-spacing: 0; }

/* ---------------------------------------------------------- sections */
.objectives, .core-concepts, .derivation, .examples, .pitfalls, .practice, .simulation, .summary { margin-bottom: 1.5em; }
.objectives ul { padding-left: 1.2em; }
.objectives li { margin: 0.35em 0; }

/* graph-paper formula block — the signature surface */
.formula {
  display: block; margin: 1.1em 0; padding: 0.95em 1.15em;
  background-color: var(--paper-elev);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
  border-left: 3px solid var(--indigo);
  border-radius: 0 var(--radius) var(--radius) 0;
  overflow-x: auto;
}

/* callout boxes */
.key-terms {
  margin: 1.2em 0; padding: 0.9em 1.15em; border-radius: var(--radius);
  background: var(--indigo-soft); border: 1px solid color-mix(in srgb, var(--indigo) 35%, transparent);
  font-size: 0.93em;
}
.key-terms::before { content: "🔑 Key Terms"; display: block; font-family: var(--font-display); font-weight: 700; font-size: 0.72rem; text-transform: uppercase; letter-spacing: 0.06em; margin-bottom: 0.35em; color: var(--indigo-deep); }
[data-theme="dark"] .key-terms::before { color: var(--indigo); }
.note {
  margin: 1.2em 0; padding: 0.9em 1.15em; border-radius: var(--radius);
  background: var(--sage-soft); border-left: 4px solid var(--sage); font-size: 0.95em;
}
.note::before { content: "💡 提示 Note"; display: block; font-family: var(--font-display); font-weight: 700; font-size: 0.72rem; margin-bottom: 0.35em; color: var(--sage); }
.pitfalls {
  background: var(--rose-soft); border: 1px solid color-mix(in srgb, var(--rose) 35%, transparent);
  padding: 0.95em 1.15em; border-radius: var(--radius);
}
.pitfalls > h2, .pitfalls > p:first-child { margin-top: 0; }
.pitfalls ul { padding-left: 1.2em; }
.pitfalls li { margin: 0.32em 0; }

/* worked examples */
.examples > h2 + p, .example { }
.example {
  margin: 1.35em 0; padding: 1.15em 1.25em;
  background: var(--paper-elev);
  border: 1px solid var(--border); border-left: 3px solid var(--ink-soft);
  border-radius: var(--radius);
}
.example h4 { margin: 0 0 0.5em; font-size: 1.02rem; color: var(--ink); }
.example-statement { font-style: italic; color: var(--ink-soft); margin-bottom: 0.8em; border-left: 2px solid var(--rule); padding-left: 0.7em; }
.example-solution { margin-top: 0.5em; }
.example-solution .step { margin: 0.6em 0; }
.ans { display: inline-block; padding: 0.05em 0.5em; background: var(--amber-soft); border: 1px solid color-mix(in srgb, var(--amber) 45%, transparent); border-radius: var(--radius-sm); font-weight: 600; color: var(--amber-deep); font-family: var(--font-display); font-size: 0.92em; }
[data-theme="dark"] .ans { color: var(--amber); }

/* practice problems */
.problem { margin: 0.8em 0; border: 1px solid var(--border); border-radius: var(--radius); overflow: hidden; background: var(--paper-elev); }
.problem > summary {
  cursor: pointer; padding: 0.75em 1em; background: var(--paper-elev); font-weight: 600;
  font-family: var(--font-display); font-size: 0.95rem; list-style: none; color: var(--ink);
}
.problem > summary::-webkit-details-marker { display: none; }
.problem > summary::before { content: "▸"; color: var(--indigo); font-size: 0.8em; margin-right: 0.5em; transition: transform .15s; display: inline-block; }
.problem[open] > summary::before { transform: rotate(90deg); }
.problem .hint, .problem .solution { padding: 0.85em 1.15em; }
.problem .hint { border-top: 1px dashed var(--rule); background: var(--amber-soft); font-size: 0.93em; }
.problem .hint::before { content: "💡 提示"; display: block; font-family: var(--font-display); font-weight: 700; font-size: 0.7rem; color: var(--amber-deep); margin-bottom: 0.25em; letter-spacing: 0.04em; }
[data-theme="dark"] .problem .hint::before { color: var(--amber); }
.problem .solution { border-top: 1px solid var(--rule); }

/* simulation */
.simulation h3 { margin-bottom: 0.4em; }
.sim-desc { color: var(--ink-soft); font-size: 0.92em; margin-top: 0; font-style: italic; }
.sim-container {
  margin: 1.1em 0; padding: 16px;
  background: var(--paper-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  display: flex; flex-direction: column; gap: 12px; align-items: center;
}
.sim-container canvas { max-width: 100%; height: auto; background: var(--sim-bg); border: 1px solid var(--border); border-radius: var(--radius-sm); touch-action: none; }
.sim-controls { width: 100%; }
.sim-controls-inner { display: flex; flex-wrap: wrap; gap: 12px 22px; align-items: center; }
.ctrl { font-family: var(--font-display); font-size: 0.84rem; color: var(--ink-soft); display: flex; flex-direction: column; gap: 4px; min-width: 150px; flex: 1 1 200px; }
.ctrl-top { display: flex; justify-content: space-between; }
.ctrl-label { font-weight: 500; }
.ctrl-value { font-weight: 700; color: var(--indigo-deep); font-variant-numeric: tabular-nums; }
[data-theme="dark"] .ctrl-value { color: var(--indigo); }
.ctrl input[type="range"] { width: 100%; accent-color: var(--indigo); height: 4px; }
.ctrl-toggle, .ctrl-button-wrap { flex-direction: row; align-items: center; gap: 6px; min-width: auto; }
.ctrl-toggle input { accent-color: var(--indigo); width: 16px; height: 16px; }
.ctrl-button-wrap { }
.ctrl-button { padding: 6px 14px; font-family: var(--font-display); font-size: 0.82rem; font-weight: 500; cursor: pointer; background: var(--indigo-soft); color: var(--indigo-deep); border: 1px solid color-mix(in srgb, var(--indigo) 40%, transparent); border-radius: var(--radius-sm); transition: background .15s, color .15s; }
[data-theme="dark"] .ctrl-button { color: var(--indigo); }
.ctrl-button:hover { background: var(--indigo); color: var(--paper-elev); }

/* summary */
.summary { background: var(--paper-elev); border: 1px solid var(--border); border-radius: var(--radius); padding: 0.5em 1.35em 0.7em; }
.summary ul { padding-left: 1.2em; }

/* chapter nav */
.chapter-nav {
  display: flex; justify-content: space-between; gap: 1em; margin-top: 3.2em;
  padding-top: 1.6em; border-top: 1px solid var(--rule);
  font-family: var(--font-display); font-size: 0.9rem;
}
.chapter-nav a { padding: 10px 15px; border: 1px solid var(--border); border-radius: var(--radius); color: var(--ink-soft); text-decoration: none; max-width: 44%; transition: border-color .15s, color .15s, transform .15s; }
.chapter-nav a.has-link:hover { border-color: var(--indigo); color: var(--indigo-deep); text-decoration: none; transform: translateY(-1px); }
[data-theme="dark"] .chapter-nav a.has-link:hover { color: var(--indigo); }
.chapter-nav a:not(.has-link) { visibility: hidden; }
.chapter-nav .next { text-align: right; margin-left: auto; }

/* ------------------------------------------------------------- index */
.index-hero { margin-bottom: 2em; }
.index-hero h1 { font-size: 2.8rem; margin-bottom: 0.25em; letter-spacing: -0.025em; font-weight: 600; }
.index-hero .en-title { font-style: italic; }
.index-hero .subtitle { font-size: 1.15rem; color: var(--ink-soft); font-style: italic; }
.roadmap {
  margin: 1.3em 0 0.5em; padding: 1em 1.2em;
  background: var(--paper-elev);
  background-image: linear-gradient(var(--grid) 1px, transparent 1px), linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
  border: 1px solid var(--border); border-left: 3px solid var(--amber); border-radius: var(--radius);
  font-size: 0.95rem;
}
.roadmap p { margin: 0.4em 0; }
.roadmap .progress { color: var(--amber-deep); font-family: var(--font-display); font-size: 0.86rem; font-weight: 600; }
[data-theme="dark"] .roadmap .progress { color: var(--amber); }
.toc-part-heading { font-family: var(--font-display); font-size: 1rem; font-weight: 700; margin: 2.2em 0 0.7em; padding-bottom: 0.35em; border-bottom: 1px solid var(--rule); color: var(--ink); letter-spacing: -0.005em; }
.toc-part-heading.upcoming { color: var(--ink-mute); border-bottom-style: dashed; }
.toc-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(285px, 1fr)); gap: 1em; margin-top: 1em; }
.toc-card {
  display: block; padding: 1em 1.15em; background: var(--paper-elev);
  border: 1px solid var(--border); border-radius: var(--radius); color: var(--ink);
  text-decoration: none; transition: border-color .16s, transform .16s, box-shadow .16s;
}
.toc-card:hover { border-color: var(--indigo); box-shadow: 0 4px 16px -8px rgba(40,35,80,0.25); text-decoration: none; transform: translateY(-2px); }
.toc-card .tc-num { font-family: var(--font-display); font-weight: 700; color: var(--amber-deep); font-size: 0.78rem; }
[data-theme="dark"] .toc-card .tc-num { color: var(--amber); }
.toc-card .tc-title { font-family: var(--font-display); font-weight: 600; color: var(--ink); font-size: 1.04rem; margin: 0.15em 0; }
.toc-card .tc-en { font-size: 0.8rem; color: var(--ink-mute); font-family: var(--font-display); font-weight: 400; }

.upcoming-list { margin-top: 0.6em; display: grid; grid-template-columns: repeat(auto-fill, minmax(300px, 1fr)); gap: 0.6em; }
.up-item { padding: 0.75em 1em; background: var(--paper-sunk); border: 1px dashed var(--border); border-radius: var(--radius); font-size: 0.92rem; color: var(--ink-soft); }
.up-item .up-sem { font-family: var(--font-display); font-weight: 700; color: var(--ink); margin-right: 0.5em; }
.up-item .up-en { display: block; font-size: 0.78rem; color: var(--ink-mute); margin-top: 0.2em; }

/* ------------------------------------------------------ focus + motion */
:focus-visible { outline: 2px solid var(--indigo); outline-offset: 2px; border-radius: 3px; }
a, button, .toc-card, .sidebar-link, .ctrl-button, .problem > summary { outline-width: 0; }
@keyframes fadeUp { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: none; } }
.content { animation: fadeUp .35s ease-out; }
@media (prefers-reduced-motion: reduce) {
  * { animation: none !important; transition: none !important; scroll-behavior: auto !important; }
}

/* ------------------------------------------------------------ responsive */
@media (max-width: 900px) {
  .sidebar { transform: translateX(-100%); transition: transform .22s ease; box-shadow: 2px 0 18px rgba(0,0,0,0.18); width: 84vw; max-width: 320px; }
  .sidebar.open { transform: translateX(0); }
  .content { margin-left: 0; padding: 62px 20px 100px; animation: none; }
  .sidebar-toggle { display: block; }
  .chapter-header h1 { font-size: 1.75rem; }
  .index-hero h1 { font-size: 1.9rem; }
  body { font-size: 17px; }
}