/* TCZ Explained v0.2 — learningBOX 風の自学習サイト */

:root {
  --bg: #fafaf7;
  --bg-elev: #ffffff;
  --bg-sidebar: #f4f3ee;
  --bg-lesson: #2a3340;
  --ink: #1a1a1a;
  --ink-muted: #6b6b6b;
  --ink-faint: #a0a0a0;
  --accent: #2a5d8f;
  --accent-light: #4a86c4;
  --accent-soft: #cfe0f3;
  --done: #5a9b6e;
  --done-soft: #d8ebde;
  --learning: #d8932b;
  --learning-soft: #fce8c4;
  --border: #e6e4dd;
  --border-strong: #d0cec5;
  --shadow: 0 1px 3px rgba(0,0,0,0.06);
  --shadow-lg: 0 4px 16px rgba(0,0,0,0.08);
  --radius: 8px;
  --radius-lg: 12px;
}

* { box-sizing: border-box; margin: 0; padding: 0; }

html, body {
  font-family: 'Inter', 'Noto Sans JP', 'Hiragino Kaku Gothic ProN', 'Yu Gothic', sans-serif;
  font-weight: 400;
  font-size: 16px;
  line-height: 1.75;
  color: var(--ink);
  background: var(--bg);
  -webkit-font-smoothing: antialiased;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

/* ====== Top header ====== */
.site-header {
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  padding: 10px 24px;
  position: sticky;
  top: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  gap: 16px;
  height: 56px;
}
.site-title {
  font-weight: 700;
  font-size: 18px;
  letter-spacing: 0.02em;
  color: var(--ink);
  white-space: nowrap;
}
.site-title a { color: inherit; text-decoration: none; }
.site-subtitle {
  font-size: 12px;
  color: var(--ink-muted);
  font-weight: 300;
  letter-spacing: 0.05em;
  white-space: nowrap;
}
.progress-bar {
  flex: 1;
  height: 6px;
  background: var(--border);
  border-radius: 3px;
  overflow: hidden;
  max-width: 320px;
}
.progress-bar-fill {
  height: 100%;
  background: var(--done);
  width: 0%;
  transition: width 400ms ease;
}
.progress-text {
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
  color: var(--ink-muted);
  letter-spacing: 0.05em;
  white-space: nowrap;
  font-weight: 500;
}
.menu-toggle {
  display: none;
  background: none;
  border: none;
  font-size: 22px;
  cursor: pointer;
  color: var(--ink);
}
.header-nav-arrows {
  display: flex;
  gap: 6px;
  margin-left: auto;
}
.nav-arrow {
  width: 32px;
  height: 32px;
  border-radius: 6px;
  border: 1px solid var(--border);
  background: var(--bg-elev);
  color: var(--ink-muted);
  cursor: pointer;
  font-size: 16px;
  display: flex;
  align-items: center;
  justify-content: center;
  text-decoration: none;
  transition: all 150ms;
}
.nav-arrow:hover {
  border-color: var(--accent);
  color: var(--accent);
  text-decoration: none;
}
.nav-arrow.disabled {
  opacity: 0.3;
  pointer-events: none;
}

/* ====== Layout ====== */
.layout {
  display: grid;
  grid-template-columns: 280px 1fr;
  min-height: calc(100vh - 56px);
}

/* ====== Sidebar ====== */
.sidebar {
  background: var(--bg-sidebar);
  border-right: 1px solid var(--border);
  padding: 16px 0;
  position: sticky;
  top: 56px;
  height: calc(100vh - 56px);
  overflow-y: auto;
}
.sidebar-thumb {
  margin: 0 16px 16px;
  padding: 12px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  display: flex;
  flex-direction: column;
  gap: 4px;
}
.sidebar-thumb .label {
  font-size: 11px;
  letter-spacing: 0.1em;
  color: var(--ink-faint);
  text-transform: uppercase;
}
.sidebar-thumb .name {
  font-weight: 600;
  font-size: 13px;
  line-height: 1.4;
  color: var(--ink);
}
.sidebar-thumb .meta {
  font-size: 11px;
  color: var(--ink-muted);
  margin-top: 4px;
}

.lesson-block {
  margin-bottom: 16px;
}
.lesson-header {
  background: var(--bg-lesson);
  color: white;
  padding: 8px 16px;
  font-size: 12px;
  font-weight: 600;
  letter-spacing: 0.02em;
  margin: 0;
}
.lesson-header .lesson-id {
  font-family: 'JetBrains Mono', monospace;
  margin-right: 6px;
  color: rgba(255,255,255,0.7);
  font-weight: 500;
}
.ch-list {
  list-style: none;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
}
.ch-list li {
  border-bottom: 1px solid var(--border);
}
.ch-list li:last-child { border-bottom: none; }
.ch-link {
  display: block;
  padding: 12px 16px 10px;
  color: var(--ink);
  text-decoration: none;
  transition: background 150ms;
  font-size: 13px;
  line-height: 1.45;
  position: relative;
}
.ch-link:hover {
  background: var(--bg-sidebar);
  text-decoration: none;
}
.ch-link.active {
  background: var(--accent-soft);
}
.ch-link.active::before {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
  bottom: 0;
  width: 3px;
  background: var(--accent);
}
.ch-link .ch-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 10px;
  letter-spacing: 0.1em;
  color: var(--ink-faint);
  margin-bottom: 2px;
}
.ch-link .ch-title {
  font-weight: 500;
  display: block;
  margin-bottom: 4px;
}
.ch-link.active .ch-title { color: var(--accent); font-weight: 600; }

.ch-status {
  font-size: 10px;
  padding: 2px 6px;
  border-radius: 3px;
  display: inline-block;
  letter-spacing: 0.05em;
  font-weight: 500;
}
.ch-status.unread {
  background: var(--bg-sidebar);
  color: var(--ink-faint);
}
.ch-status.viewing {
  background: var(--accent);
  color: white;
}
.ch-status.completed {
  background: var(--done-soft);
  color: #2d6a47;
}

.sidebar-extras {
  border-top: 1px solid var(--border-strong);
  margin-top: 12px;
  padding-top: 12px;
}
.sidebar-extras .ch-link {
  font-size: 12px;
  color: var(--ink-muted);
}

.reset-btn {
  margin: 16px;
  padding: 6px 12px;
  background: transparent;
  border: 1px solid var(--border-strong);
  border-radius: var(--radius);
  font-size: 11px;
  color: var(--ink-muted);
  cursor: pointer;
  letter-spacing: 0.05em;
  width: calc(100% - 32px);
}
.reset-btn:hover { background: var(--bg-elev); }

/* ====== Breadcrumb ====== */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 12px;
  color: var(--ink-muted);
  margin-bottom: 16px;
  padding-bottom: 16px;
  border-bottom: 1px solid var(--border);
}
.breadcrumb a {
  color: var(--ink-muted);
  text-decoration: none;
}
.breadcrumb a:hover { color: var(--accent); }
.breadcrumb .separator { color: var(--ink-faint); }
.breadcrumb .current { color: var(--ink); font-weight: 500; }

/* ====== Main ====== */
.main {
  padding: 32px 56px 96px;
  max-width: 880px;
  margin: 0 auto;
  width: 100%;
}

.chapter-meta {
  display: flex;
  gap: 12px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.08em;
  color: var(--ink-faint);
  text-transform: uppercase;
  margin-bottom: 12px;
  flex-wrap: wrap;
  align-items: center;
}
.chapter-meta .chapter-num {
  color: var(--accent);
  font-weight: 600;
}
.chapter-meta .tag-badge {
  background: var(--accent-soft);
  color: var(--accent);
  padding: 2px 8px;
  border-radius: 10px;
  font-size: 10px;
  text-transform: none;
  letter-spacing: 0.02em;
}

article > header h1 {
  font-size: 30px;
  font-weight: 600;
  letter-spacing: -0.005em;
  line-height: 1.3;
  margin-bottom: 12px;
  color: var(--ink);
}

.lead {
  font-size: 16px;
  color: var(--ink-muted);
  line-height: 1.75;
  margin-bottom: 32px;
  font-weight: 300;
  border-left: 2px solid var(--border-strong);
  padding-left: 16px;
}

article h2 {
  font-size: 22px;
  font-weight: 600;
  margin: 48px 0 12px;
  padding-bottom: 6px;
  border-bottom: 1px solid var(--border);
  letter-spacing: -0.005em;
  color: var(--ink);
}

article h3 {
  font-size: 17px;
  font-weight: 600;
  margin: 28px 0 8px;
  color: var(--ink);
}

article p { margin: 12px 0; }
article ul, article ol { margin: 12px 0; padding-left: 24px; }
article li { margin: 6px 0; }
article strong { font-weight: 600; }
article em { font-style: italic; }

article blockquote {
  border-left: 3px solid var(--accent-soft);
  background: var(--bg-elev);
  padding: 12px 20px;
  margin: 20px 0;
  color: var(--ink);
  border-radius: 0 var(--radius) var(--radius) 0;
}

article code {
  font-family: 'JetBrains Mono', monospace;
  font-size: 0.88em;
  background: var(--bg-sidebar);
  padding: 2px 6px;
  border-radius: 3px;
  color: var(--accent);
}

article pre {
  background: var(--bg-sidebar);
  border: 1px solid var(--border);
  padding: 14px 18px;
  margin: 16px 0;
  border-radius: var(--radius);
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  line-height: 1.6;
  overflow-x: auto;
}
article pre code { background: none; padding: 0; color: var(--ink); }

article table {
  border-collapse: collapse;
  margin: 16px 0;
  font-size: 14px;
  width: 100%;
}
article th, article td {
  border: 1px solid var(--border);
  padding: 8px 12px;
  text-align: left;
  vertical-align: top;
  line-height: 1.6;
}
article th { background: var(--bg-sidebar); font-weight: 600; }

article hr { border: none; border-top: 1px solid var(--border); margin: 32px 0; }

mjx-container { font-size: 1em !important; }
mjx-container[display="true"] {
  margin: 16px 0 !important;
  padding: 8px 0;
  overflow-x: auto;
}

/* ====== Callouts ====== */
.callout {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 4px solid var(--accent);
  padding: 16px 20px;
  margin: 20px 0;
  border-radius: 0 var(--radius) var(--radius) 0;
}
.callout-title {
  font-weight: 600;
  font-size: 13px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent);
  margin-bottom: 8px;
}
.callout.note { border-left-color: var(--accent); }
.callout.note .callout-title { color: var(--accent); }
.callout.warn { border-left-color: #c4452f; background: #fdf6f4; }
.callout.warn .callout-title { color: #8a3020; }
.callout.tip  { border-left-color: var(--done); background: var(--done-soft); }
.callout.tip  .callout-title { color: #2d6a47; }

/* ====== Check questions ====== */
.check-block {
  background: #fffaeb;
  border: 1px solid #f0d779;
  border-radius: var(--radius);
  padding: 16px 20px;
  margin: 24px 0;
}
.check-block h4 {
  font-size: 13px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: #8a6c1a;
  margin-bottom: 8px;
  font-weight: 700;
}
.check-block details { margin-top: 8px; }
.check-block summary {
  cursor: pointer;
  color: var(--accent);
  font-size: 13px;
  font-weight: 500;
  padding: 4px 0;
  user-select: none;
}
.check-block summary:hover { text-decoration: underline; }
.check-block .answer {
  margin-top: 8px;
  padding: 12px;
  background: var(--bg-elev);
  border-radius: 4px;
  font-size: 14px;
  color: var(--ink-muted);
  line-height: 1.7;
}

/* ====== Chapter footer ====== */
.chapter-nav {
  margin-top: 64px;
  padding-top: 32px;
  border-top: 1px solid var(--border);
  display: grid;
  grid-template-columns: 1fr auto 1fr;
  align-items: center;
  gap: 16px;
}
.chapter-nav .prev-link,
.chapter-nav .next-link {
  display: flex;
  flex-direction: column;
  padding: 12px 16px;
  border-radius: var(--radius);
  font-size: 14px;
  color: var(--ink);
  background: var(--bg-elev);
  border: 1px solid var(--border);
  text-decoration: none;
  transition: all 150ms;
}
.chapter-nav .prev-link:hover,
.chapter-nav .next-link:hover {
  border-color: var(--accent);
  text-decoration: none;
  box-shadow: var(--shadow);
}
.chapter-nav .next-link { text-align: right; }
.chapter-nav .nav-label {
  font-size: 11px;
  color: var(--ink-faint);
  letter-spacing: 0.1em;
  text-transform: uppercase;
}
.chapter-nav .nav-title { font-weight: 500; margin-top: 2px; }

.complete-btn {
  padding: 14px 28px;
  background: var(--accent);
  color: white;
  border: none;
  border-radius: var(--radius);
  font-size: 14px;
  font-weight: 600;
  cursor: pointer;
  transition: all 150ms;
  letter-spacing: 0.02em;
  white-space: nowrap;
}
.complete-btn:hover { background: #1f4870; box-shadow: var(--shadow-lg); }
.complete-btn.done { background: var(--done); }
.complete-btn.done:hover { background: #4a8a5e; }
.complete-btn::before { content: "○ "; margin-right: 4px; }
.complete-btn.done::before { content: "✓ "; }

/* ====== Index page ====== */
.hero {
  padding: 48px 32px 56px;
  text-align: center;
  background: linear-gradient(180deg, var(--bg-elev), transparent);
  margin: -32px -56px 32px;
  border-bottom: 1px solid var(--border);
}
.hero h1 {
  font-size: 40px;
  font-weight: 700;
  letter-spacing: -0.02em;
  margin-bottom: 12px;
}
.hero .ja-sub {
  font-size: 16px;
  color: var(--ink-muted);
  font-weight: 300;
  margin-bottom: 24px;
}
.hero .start-btn {
  display: inline-block;
  padding: 14px 32px;
  background: var(--accent);
  color: white;
  border-radius: var(--radius);
  font-size: 15px;
  font-weight: 500;
  text-decoration: none;
  transition: all 150ms;
}
.hero .start-btn:hover { background: #1f4870; text-decoration: none; box-shadow: var(--shadow-lg); }

/* Lesson sections in index */
.lesson-section {
  margin: 32px 0 40px;
}
.lesson-section-header {
  display: flex;
  align-items: baseline;
  gap: 12px;
  margin-bottom: 16px;
  padding-bottom: 8px;
  border-bottom: 2px solid var(--bg-lesson);
}
.lesson-section-header .lesson-id {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.15em;
  color: var(--ink-faint);
  font-weight: 600;
}
.lesson-section-header h2 {
  font-size: 18px;
  font-weight: 700;
  margin: 0;
  border: none;
  padding: 0;
}
.lesson-section-header .lesson-subtitle {
  font-size: 13px;
  color: var(--ink-muted);
  font-weight: 400;
}

.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 16px;
}

.chapter-card {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 16px 18px 18px;
  text-decoration: none;
  color: var(--ink);
  transition: all 200ms;
  display: flex;
  flex-direction: column;
  gap: 8px;
  position: relative;
  overflow: hidden;
}
.chapter-card:hover {
  border-color: var(--accent);
  box-shadow: var(--shadow-lg);
  text-decoration: none;
  transform: translateY(-1px);
}
.chapter-card .card-top {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 8px;
}
.chapter-card .num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.15em;
  color: var(--accent);
  font-weight: 600;
}
.chapter-card .tag {
  font-size: 10px;
  background: var(--bg-sidebar);
  color: var(--ink-muted);
  padding: 2px 8px;
  border-radius: 10px;
  font-weight: 500;
}
.chapter-card .title {
  font-weight: 600;
  line-height: 1.4;
  font-size: 15px;
  color: var(--ink);
}
.chapter-card .meta {
  font-size: 12px;
  color: var(--ink-muted);
  margin-top: 4px;
}
.chapter-card .progress-row {
  display: flex;
  align-items: center;
  gap: 8px;
  margin-top: 8px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 10px;
  letter-spacing: 0.05em;
}
.chapter-card .progress-bar-mini {
  flex: 1;
  height: 4px;
  background: var(--border);
  border-radius: 2px;
  overflow: hidden;
}
.chapter-card .progress-bar-mini-fill {
  height: 100%;
  width: 0%;
  background: var(--accent-light);
  transition: width 400ms ease;
}
.chapter-card .progress-pct { color: var(--ink-muted); font-weight: 500; }

.chapter-card .action {
  margin-top: 8px;
  padding: 8px 12px;
  background: var(--accent);
  color: white;
  border-radius: 6px;
  font-size: 13px;
  font-weight: 500;
  text-align: center;
  letter-spacing: 0.02em;
}

.chapter-card.completed {
  background: linear-gradient(180deg, var(--done-soft), var(--bg-elev));
}
.chapter-card.completed .progress-bar-mini-fill { background: var(--done); width: 100% !important; }
.chapter-card.completed .progress-pct { color: var(--done); }
.chapter-card.completed .action { background: var(--done); }

.chapter-card.viewing { border-color: var(--accent); }

/* ====== Top index: course selector ====== */
.section-h {
  font-size: 18px;
  font-weight: 700;
  margin: 32px 0 12px;
  padding-bottom: 8px;
  border-bottom: 2px solid var(--bg-lesson);
}
.course-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 24px;
  margin: 24px 0 48px;
}
.course-card {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 28px 28px 24px;
  text-decoration: none;
  color: var(--ink);
  transition: all 200ms;
  display: flex;
  flex-direction: column;
  gap: 10px;
  position: relative;
}
.course-card:hover {
  border-color: var(--accent);
  box-shadow: var(--shadow-lg);
  text-decoration: none;
  transform: translateY(-2px);
}
.course-card-tag {
  display: inline-block;
  align-self: flex-start;
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.15em;
  background: var(--accent);
  color: white;
  padding: 4px 12px;
  border-radius: 12px;
  font-weight: 600;
  text-transform: uppercase;
}
.course-card-title {
  font-size: 22px;
  font-weight: 700;
  line-height: 1.3;
  color: var(--ink);
  margin-top: 4px;
}
.course-card-desc {
  font-size: 14px;
  color: var(--ink-muted);
  line-height: 1.7;
  margin: 4px 0;
}
.course-card-meta {
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
  color: var(--ink-faint);
  letter-spacing: 0.05em;
}
.course-card-action {
  margin-top: 12px;
  padding: 12px 16px;
  background: var(--accent);
  color: white;
  border-radius: var(--radius);
  text-align: center;
  font-size: 14px;
  font-weight: 500;
  letter-spacing: 0.02em;
}
.course-card:hover .course-card-action { background: #1f4870; }

/* ====== Extras grid (top page) ====== */
.extra-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 16px;
  margin: 24px 0 48px;
}
.extra-card {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 20px 24px;
  text-decoration: none;
  color: var(--ink);
  transition: all 200ms;
  display: grid;
  grid-template-columns: 56px 1fr auto;
  align-items: center;
  gap: 16px;
}
.extra-card:hover {
  border-color: var(--accent);
  box-shadow: var(--shadow-lg);
  text-decoration: none;
  transform: translateY(-1px);
}
.extra-card-icon {
  width: 56px;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--bg-sidebar);
  border-radius: var(--radius);
  font-size: 22px;
  font-weight: 700;
  color: var(--accent);
  font-family: 'JetBrains Mono', monospace;
}
.extra-card-body {
  min-width: 0;
}
.extra-card-title {
  font-size: 16px;
  font-weight: 700;
  margin-bottom: 4px;
  color: var(--ink);
}
.extra-card-desc {
  font-size: 13px;
  color: var(--ink-muted);
  line-height: 1.6;
  margin: 4px 0;
}
.extra-card-meta {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  color: var(--ink-faint);
  letter-spacing: 0.05em;
  margin-top: 4px;
}
.extra-card-arrow {
  font-size: 18px;
  color: var(--accent);
  font-weight: 600;
  transition: transform 200ms;
}
.extra-card:hover .extra-card-arrow {
  transform: translateX(4px);
}

@media (max-width: 600px) {
  .extra-card {
    grid-template-columns: 48px 1fr;
    gap: 12px;
    padding: 16px 18px;
  }
  .extra-card-icon { width: 48px; height: 48px; font-size: 18px; }
  .extra-card-arrow { display: none; }
}

.hero-tag {
  display: inline-block;
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.2em;
  background: var(--accent);
  color: white;
  padding: 4px 14px;
  border-radius: 12px;
  font-weight: 600;
  text-transform: uppercase;
  margin-bottom: 12px;
}

/* ====== Diagrams (inline SVG) ====== */
.diagram-wrap {
  margin: 20px 0;
  padding: 16px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  text-align: center;
}
.diagram-wrap svg {
  display: block;
  margin: 0 auto;
  max-width: 100%;
  height: auto;
}
.diagram-caption {
  text-align: center;
  font-size: 12px;
  color: var(--ink-muted);
  margin-top: 8px;
  letter-spacing: 0.02em;
  font-weight: 400;
}

/* ====== QA page ====== */
.qa-toc {
  background: var(--bg-sidebar);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 16px 20px;
  margin: 16px 0 32px;
  font-size: 14px;
}
.qa-toc h3 {
  font-size: 13px;
  font-weight: 600;
  margin: 0 0 8px;
  letter-spacing: 0.02em;
}
.qa-toc ol {
  margin: 0;
  padding-left: 20px;
  columns: 2;
  column-gap: 24px;
}
.qa-toc li { margin: 3px 0; line-height: 1.5; }
.qa-toc a { color: var(--accent); text-decoration: none; }
.qa-toc a:hover { text-decoration: underline; }

.qa-block {
  margin: 32px 0;
  padding: 20px 24px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 4px solid var(--accent);
  border-radius: 0 var(--radius) var(--radius) 0;
}
.qa-block h3 {
  font-size: 16px;
  font-weight: 600;
  margin: 0 0 12px;
  color: var(--accent);
}
.qa-block h3::before {
  content: "Q. ";
  font-family: 'JetBrains Mono', monospace;
  font-weight: 700;
}
.qa-block .answer {
  font-size: 14.5px;
  line-height: 1.8;
}
.qa-block .answer::before {
  content: "A. ";
  font-family: 'JetBrains Mono', monospace;
  font-weight: 700;
  color: var(--done);
}

.qa-section-h {
  font-size: 18px;
  font-weight: 700;
  margin: 48px 0 16px;
  padding: 8px 16px;
  background: var(--bg-lesson);
  color: white;
  border-radius: var(--radius);
  letter-spacing: 0.02em;
}

/* ====== Footer ====== */
.site-footer {
  margin-top: 80px;
  padding: 24px 32px;
  border-top: 1px solid var(--border);
  font-size: 12px;
  color: var(--ink-faint);
  text-align: center;
  letter-spacing: 0.05em;
}
.site-footer a { color: var(--ink-muted); }

/* ====== Responsive ====== */
@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar {
    position: fixed;
    top: 56px;
    left: -100%;
    width: 80%;
    max-width: 320px;
    height: calc(100vh - 56px);
    z-index: 200;
    transition: left 250ms;
    box-shadow: 4px 0 12px rgba(0,0,0,0.1);
  }
  .sidebar.open { left: 0; }
  .menu-toggle { display: block; }
  .main { padding: 24px 20px 64px; }
  .hero { margin: -24px -20px 24px; padding: 36px 20px 40px; }
  .hero h1 { font-size: 28px; }
  article > header h1 { font-size: 24px; }
  .chapter-nav { grid-template-columns: 1fr; }
  .header-nav-arrows { display: none; }
}