/* ═══════════════════════════════════════════════════════════
   EngineeringOS — Shared Design System
   Modern Learning Platform v2
   ═══════════════════════════════════════════════════════════ */

:root {
  --bg:        #0d0d14;
  --bg2:       #11111a;
  --surface:   #17172200;
  --s1:        #171722;
  --s2:        #1e1e2d;
  --s3:        #252538;
  --border:    #2a2a3e;
  --border2:   #363650;

  --teal:      #00c8a0;
  --teal-d:    rgba(0,200,160,0.08);
  --teal-b:    rgba(0,200,160,0.18);
  --orange:    #ff6b35;
  --orange-d:  rgba(255,107,53,0.08);
  --orange-b:  rgba(255,107,53,0.18);
  --blue:      #5b8fff;
  --blue-d:    rgba(91,143,255,0.08);
  --blue-b:    rgba(91,143,255,0.18);
  --purple:    #a78bfa;
  --purple-d:  rgba(167,139,250,0.08);
  --amber:     #fbbf24;
  --amber-d:   rgba(251,191,36,0.08);
  --green:     #34d399;
  --red:       #f87171;

  --text:      #e2e8f0;
  --text2:     #94a3b8;
  --muted:     #64748b;

  --easy-c:    #22c55e;
  --med-c:     #eab308;
  --hard-c:    #f97316;

  --font-body: 'Inter', sans-serif;
  --font-head: 'Syne', sans-serif;
  --font-code: 'JetBrains Mono', monospace;
}

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

html { scroll-behavior: smooth; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-body);
  font-size: 15px;
  line-height: 1.7;
  -webkit-font-smoothing: antialiased;
}

/* ── Nav ── */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  height: 58px;
  background: rgba(13,13,20,0.88);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 2rem;
  gap: 1rem;
}
.nav-logo {
  font-family: var(--font-head);
  font-size: 1rem;
  font-weight: 800;
  color: var(--text);
  text-decoration: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}
.nav-logo span { color: var(--teal); }
.nav-links { display: flex; align-items: center; gap: 0.25rem; }
.nav-link {
  font-family: var(--font-code);
  font-size: 0.72rem;
  color: var(--muted);
  text-decoration: none;
  padding: 0.35rem 0.75rem;
  border-radius: 6px;
  transition: all 0.15s;
  letter-spacing: 0.03em;
}
.nav-link:hover { color: var(--text); background: var(--s2); }
.nav-link.active { color: var(--teal); background: var(--teal-d); }
.back-link {
  font-family: var(--font-code);
  font-size: 0.75rem;
  color: var(--muted);
  text-decoration: none;
  display: flex;
  align-items: center;
  gap: 0.4rem;
  padding: 0.35rem 0.75rem;
  border-radius: 6px;
  transition: all 0.15s;
}
.back-link:hover { color: var(--teal); background: var(--teal-d); }
.nav-badge {
  font-family: var(--font-code);
  font-size: 0.72rem;
  color: var(--teal);
  letter-spacing: 0.04em;
}

/* ── Hero ── */
.page-hero {
  max-width: 1140px;
  margin: 0 auto;
  padding: 4rem 2rem 2.5rem;
}
.page-hero .eyebrow {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  font-family: var(--font-code);
  font-size: 0.7rem;
  letter-spacing: 0.1em;
  color: var(--teal);
  background: var(--teal-d);
  border: 1px solid var(--teal-b);
  padding: 0.3rem 0.75rem;
  border-radius: 100px;
  margin-bottom: 1.5rem;
}
.page-hero h1 {
  font-family: var(--font-head);
  font-size: clamp(2rem, 5vw, 3.5rem);
  font-weight: 800;
  letter-spacing: -0.025em;
  line-height: 1.1;
  margin-bottom: 1rem;
}
.page-hero h1 em {
  font-style: normal;
  background: linear-gradient(135deg, var(--teal) 0%, var(--blue) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.page-hero p {
  font-size: 1rem;
  color: var(--text2);
  max-width: 620px;
  margin-bottom: 2.5rem;
}

/* ── Stats row ── */
.stats-row {
  display: flex;
  gap: 1rem;
  flex-wrap: wrap;
  margin-bottom: 3rem;
}
.stat {
  background: var(--s1);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 0.9rem 1.4rem;
  min-width: 110px;
}
.stat-val {
  font-family: var(--font-head);
  font-size: 1.6rem;
  font-weight: 800;
  color: var(--teal);
  line-height: 1;
  margin-bottom: 0.25rem;
}
.stat-key {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

/* ── Section nav tabs ── */
.section-tabs {
  display: flex;
  gap: 0.4rem;
  flex-wrap: wrap;
  max-width: 1140px;
  margin: 0 auto 2rem;
  padding: 0 2rem;
}
.section-tab {
  font-family: var(--font-code);
  font-size: 0.72rem;
  color: var(--muted);
  text-decoration: none;
  padding: 0.4rem 1rem;
  border: 1px solid var(--border);
  border-radius: 6px;
  background: var(--s1);
  transition: all 0.15s;
  letter-spacing: 0.03em;
  white-space: nowrap;
}
.section-tab:hover { border-color: var(--teal-b); color: var(--teal); }
.section-tab.highlight { border-color: var(--orange-b); color: var(--orange); }
.section-tab.alt { border-color: var(--blue-b); color: var(--blue); }

/* ── Main content container ── */
.content-wrap {
  max-width: 1140px;
  margin: 0 auto;
  padding: 0 2rem 6rem;
}

/* ── Section groups ── */
.content-section { margin-bottom: 4rem; }
.section-header {
  display: flex;
  align-items: center;
  gap: 1rem;
  margin-bottom: 1.5rem;
  padding-bottom: 1rem;
  border-bottom: 1px solid var(--border);
}
.section-num {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--muted);
}
.section-title {
  font-family: var(--font-head);
  font-size: 1.25rem;
  font-weight: 700;
}
.section-badge {
  margin-left: auto;
  font-family: var(--font-code);
  font-size: 0.65rem;
  padding: 0.2rem 0.6rem;
  border-radius: 4px;
  background: var(--teal-d);
  border: 1px solid var(--teal-b);
  color: var(--teal);
  letter-spacing: 0.06em;
}
.section-badge.orange { background: var(--orange-d); border-color: var(--orange-b); color: var(--orange); }
.section-badge.blue { background: var(--blue-d); border-color: var(--blue-b); color: var(--blue); }

/* ── Pattern / Topic cards ── */
.pattern-card {
  background: var(--s1);
  border: 1px solid var(--border);
  border-radius: 12px;
  margin-bottom: 1rem;
  overflow: hidden;
  transition: border-color 0.15s;
}
.pattern-card:hover { border-color: var(--border2); }
.pattern-card.open { border-color: rgba(0,200,160,0.25); }

.pattern-header {
  display: flex;
  align-items: center;
  gap: 1rem;
  padding: 1.1rem 1.5rem;
  cursor: pointer;
  user-select: none;
}
.pattern-header:hover { background: rgba(255,255,255,0.015); }
.pattern-num {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--teal);
  min-width: 48px;
  font-weight: 600;
}
.pattern-name {
  font-family: var(--font-head);
  font-size: 0.95rem;
  font-weight: 700;
  flex: 1;
}
.pattern-tag {
  font-family: var(--font-code);
  font-size: 0.62rem;
  padding: 0.2rem 0.55rem;
  border-radius: 4px;
  border: 1px solid var(--border2);
  color: var(--muted);
}
.pattern-toggle {
  color: var(--muted);
  font-size: 0.75rem;
  transition: transform 0.25s;
  flex-shrink: 0;
}
.pattern-card.open .pattern-toggle { transform: rotate(180deg); }

.pattern-body {
  display: none;
  padding: 0 1.5rem 1.5rem;
}
.pattern-body.open { display: block; }

/* ── Content grids ── */
.grid2 { display: grid; grid-template-columns: 1fr 1fr; gap: 1rem; margin-bottom: 1rem; }
.grid3 { display: grid; grid-template-columns: 1fr 1fr 1fr; gap: 1rem; margin-bottom: 1rem; }

/* ── Info blocks ── */
.info-block {
  background: var(--s2);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.1rem;
}
.info-block h4 {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--teal);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 0.75rem;
}
.info-block p, .info-block li {
  font-size: 0.875rem;
  color: var(--text2);
  line-height: 1.7;
}
.info-block ul { list-style: none; display: flex; flex-direction: column; gap: 0.35rem; }
.info-block li::before { content: '▸ '; color: var(--teal); font-size: 0.7rem; }
.info-block strong { color: var(--text); }

/* ── Callout boxes ── */
.callout {
  border-radius: 8px;
  padding: 1rem 1.1rem;
  margin-top: 0.75rem;
  border-left: 3px solid;
}
.callout h4 {
  font-family: var(--font-code);
  font-size: 0.65rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 0.5rem;
  font-weight: 600;
}
.callout p, .callout li { font-size: 0.875rem; line-height: 1.7; }
.callout ul { list-style: none; display: flex; flex-direction: column; gap: 0.3rem; }
.callout li::before { content: '▸ '; font-size: 0.7rem; }

.callout-tip    { background: var(--teal-d);   border-color: var(--teal);   }
.callout-tip h4 { color: var(--teal); }
.callout-tip p, .callout-tip li { color: var(--text2); }
.callout-tip li::before { color: var(--teal); }

.callout-warn   { background: var(--orange-d); border-color: var(--orange); }
.callout-warn h4 { color: var(--orange); }
.callout-warn p, .callout-warn li { color: var(--text2); }
.callout-warn li::before { color: var(--orange); }

.callout-info   { background: var(--blue-d);   border-color: var(--blue);   }
.callout-info h4 { color: var(--blue); }
.callout-info p, .callout-info li { color: var(--text2); }
.callout-info li::before { color: var(--blue); }

.callout-note   { background: var(--amber-d);  border-color: var(--amber);  }
.callout-note h4 { color: var(--amber); }
.callout-note p, .callout-note li { color: var(--text2); }
.callout-note li::before { color: var(--amber); }

/* ── Code blocks ── */
.code-block {
  background: #0a0a12;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1rem 1.1rem;
  margin-top: 0.75rem;
  overflow-x: auto;
}
.code-block-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 0.75rem;
  padding-bottom: 0.5rem;
  border-bottom: 1px solid var(--border);
}
.code-block-lang {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--teal);
  text-transform: uppercase;
  letter-spacing: 0.1em;
}
.code-block-label {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--muted);
}
.code-block pre {
  font-family: var(--font-code);
  font-size: 0.8rem;
  color: #c9d1d9;
  line-height: 1.75;
  white-space: pre;
  tab-size: 2;
}
.code-block pre .kw { color: #ff79c6; }     /* keywords */
.code-block pre .fn { color: #50fa7b; }     /* functions */
.code-block pre .st { color: #f1fa8c; }     /* strings */
.code-block pre .cm { color: #6272a4; }     /* comments */
.code-block pre .ty { color: #8be9fd; }     /* types */
.code-block pre .nu { color: #bd93f9; }     /* numbers */

/* ── Problem items ── */
.problem-block {
  background: var(--s2);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.1rem;
  margin-bottom: 0.75rem;
}
.problem-header {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 0.6rem;
}
.problem-name {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text);
  flex: 1;
}
.badge {
  font-family: var(--font-code);
  font-size: 0.6rem;
  padding: 0.15rem 0.5rem;
  border-radius: 100px;
  font-weight: 600;
}
.badge-easy   { background: rgba(34,197,94,0.1);  color: var(--easy-c);  border: 1px solid rgba(34,197,94,0.2); }
.badge-medium { background: rgba(234,179,8,0.1);   color: var(--med-c);   border: 1px solid rgba(234,179,8,0.2); }
.badge-hard   { background: rgba(249,115,22,0.1);  color: var(--hard-c);  border: 1px solid rgba(249,115,22,0.2); }

.problem-stmt {
  font-size: 0.85rem;
  color: var(--text2);
  margin-bottom: 0.6rem;
  line-height: 1.65;
}
.problem-insight {
  font-size: 0.82rem;
  color: var(--teal);
  background: var(--teal-d);
  border: 1px solid var(--teal-b);
  border-radius: 5px;
  padding: 0.45rem 0.75rem;
  margin-bottom: 0.5rem;
  font-family: var(--font-code);
}
.complexity {
  display: flex;
  gap: 0.75rem;
  margin-top: 0.4rem;
}
.complexity-item {
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--muted);
}
.complexity-item span { color: var(--amber); }

/* ── Tab system ── */
.tab-nav {
  display: flex;
  border-bottom: 1px solid var(--border);
  gap: 0;
  flex-wrap: wrap;
}
.tab-btn {
  font-family: var(--font-code);
  font-size: 0.72rem;
  padding: 0.7rem 1.1rem;
  background: none;
  border: none;
  color: var(--muted);
  cursor: pointer;
  transition: all 0.15s;
  letter-spacing: 0.04em;
  white-space: nowrap;
  border-bottom: 2px solid transparent;
  margin-bottom: -1px;
}
.tab-btn:hover { color: var(--text); }
.tab-btn.active { color: var(--teal); border-bottom-color: var(--teal); }
.tab-panel { display: none; padding: 1.5rem 0; }
.tab-panel.active { display: block; }

/* ── Diagrams ── */
.diagram {
  background: #09090f;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.1rem;
  font-family: var(--font-code);
  font-size: 0.75rem;
  color: var(--text2);
  line-height: 1.9;
  overflow-x: auto;
  white-space: pre;
  margin-top: 0.75rem;
}
.diagram .hl  { color: var(--teal); }
.diagram .hl2 { color: var(--orange); }
.diagram .hl3 { color: var(--blue); }
.diagram .hl4 { color: var(--amber); }

/* ── Decision/comparison table ── */
.compare-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.85rem;
  margin-top: 0.75rem;
}
.compare-table th {
  background: var(--s3);
  color: var(--text2);
  font-family: var(--font-code);
  font-size: 0.65rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  padding: 0.6rem 0.9rem;
  text-align: left;
  border-bottom: 1px solid var(--border2);
}
.compare-table td {
  padding: 0.55rem 0.9rem;
  color: var(--text2);
  border-bottom: 1px solid var(--border);
  vertical-align: top;
  line-height: 1.6;
}
.compare-table td:first-child { color: var(--text); font-weight: 500; }
.compare-table tr:last-child td { border-bottom: none; }

/* ── Index page cards ── */
.courses-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 1rem;
}
.course-card {
  background: var(--s1);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 1.5rem;
  text-decoration: none;
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
  transition: all 0.2s;
  position: relative;
  overflow: hidden;
}
.course-card::before {
  content: '';
  position: absolute;
  top: 0; left: 0; right: 0;
  height: 2px;
  background: linear-gradient(90deg, var(--card-accent, var(--teal)), transparent);
  opacity: 0;
  transition: opacity 0.2s;
}
.course-card:hover { border-color: var(--border2); transform: translateY(-1px); }
.course-card:hover::before { opacity: 1; }
.course-icon {
  font-size: 1.5rem;
  line-height: 1;
}
.course-title {
  font-family: var(--font-head);
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--text);
}
.course-desc {
  font-size: 0.85rem;
  color: var(--text2);
  line-height: 1.6;
  flex: 1;
}
.course-meta {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
}
.course-tag {
  font-family: var(--font-code);
  font-size: 0.62rem;
  padding: 0.2rem 0.55rem;
  border-radius: 4px;
  background: var(--s3);
  color: var(--muted);
  border: 1px solid var(--border);
}

/* ── Checkpoint/summary cards ── */
.checkpoint {
  background: var(--orange-d);
  border: 1px solid var(--orange-b);
  border-radius: 10px;
  padding: 1.5rem;
  margin-top: 2rem;
}
.checkpoint h3 {
  font-family: var(--font-head);
  font-size: 1rem;
  font-weight: 700;
  color: var(--orange);
  margin-bottom: 0.75rem;
}
.checkpoint ul { list-style: none; display: flex; flex-direction: column; gap: 0.4rem; }
.checkpoint li { font-size: 0.875rem; color: var(--text2); display: flex; gap: 0.5rem; }
.checkpoint li::before { content: '✓'; color: var(--orange); flex-shrink: 0; }

/* ── Pattern index card ── */
.pattern-index-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
  gap: 0.75rem;
  margin-bottom: 2rem;
}
.pattern-index-card {
  background: var(--s1);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.9rem 1.1rem;
  text-decoration: none;
  transition: all 0.15s;
}
.pattern-index-card:hover { border-color: var(--teal-b); background: var(--s2); }
.pattern-index-num { font-family: var(--font-code); font-size: 0.62rem; color: var(--teal); }
.pattern-index-name { font-size: 0.88rem; font-weight: 600; color: var(--text); margin-top: 0.2rem; }
.pattern-index-sub { font-size: 0.78rem; color: var(--muted); margin-top: 0.2rem; }

/* ── Responsive ── */
@media (max-width: 768px) {
  .grid2, .grid3 { grid-template-columns: 1fr; }
  .courses-grid { grid-template-columns: 1fr; }
  .site-nav .nav-links { display: none; }
  .page-hero { padding: 2.5rem 1rem 1.5rem; }
  .content-wrap { padding: 0 1rem 4rem; }
  .section-tabs { padding: 0 1rem; }
}