:root {
  --color-bg: #ffffff;
  --color-bg-secondary: #f8f9fb;
  --color-bg-tertiary: #f0f2f5;
  --color-text: #1a1a2e;
  --color-text-secondary: #555770;
  --color-text-tertiary: #8b8da3;
  --color-primary: #4f46e5;
  --color-primary-light: #eef2ff;
  --color-primary-dark: #3730a3;
  --color-border: #e2e4ea;
  --color-border-light: #eef0f4;
  --color-card-hover: #fafafc;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow-sm: 0 1px 2px rgba(0,0,0,0.04);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.06);
  --shadow-lg: 0 8px 24px rgba(0,0,0,0.08);
  --max-width: 960px;
  --sidebar-width: 280px;
  --transition: 0.2s ease;
}

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

html { scroll-behavior: smooth; }

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Inter', Roboto, sans-serif;
  color: var(--color-text);
  background: var(--color-bg);
  line-height: 1.65;
  font-size: 15px;
  -webkit-font-smoothing: antialiased;
}

/* ── Header ── */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(255,255,255,0.92);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--color-border-light);
}

.header-inner {
  max-width: 1200px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 24px;
  height: 56px;
}

.logo {
  display: flex;
  align-items: center;
  gap: 10px;
  text-decoration: none;
  color: var(--color-text);
  font-weight: 700;
  font-size: 16px;
}

.logo img {
  height: 24px;
  width: auto;
}

.logo span {
  color: var(--color-text-secondary);
  font-weight: 400;
  margin-left: 2px;
}

.header-search {
  position: relative;
  width: 320px;
}

.header-search input {
  width: 100%;
  padding: 8px 14px 8px 36px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  font-size: 14px;
  background: var(--color-bg-secondary);
  color: var(--color-text);
  outline: none;
  transition: border-color var(--transition), box-shadow var(--transition);
}

.header-search input:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px var(--color-primary-light);
  background: var(--color-bg);
}

.header-search input::placeholder {
  color: var(--color-text-tertiary);
}

.header-search .search-icon {
  position: absolute;
  left: 11px;
  top: 50%;
  transform: translateY(-50%);
  width: 16px;
  height: 16px;
  color: var(--color-text-tertiary);
}

.header-search .search-shortcut {
  position: absolute;
  right: 10px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 11px;
  color: var(--color-text-tertiary);
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: 4px;
  padding: 1px 6px;
  font-family: inherit;
  pointer-events: none;
}

.menu-toggle {
  display: none;
  background: none;
  border: none;
  cursor: pointer;
  padding: 6px;
  color: var(--color-text);
}

/* ── Search Results Overlay ── */
.search-results {
  display: none;
  position: absolute;
  top: calc(100% + 6px);
  left: 0;
  right: 0;
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  box-shadow: var(--shadow-lg);
  max-height: 400px;
  overflow-y: auto;
  z-index: 200;
}

.search-results.active { display: block; }

.search-result-item {
  display: block;
  padding: 10px 14px;
  text-decoration: none;
  color: var(--color-text);
  border-bottom: 1px solid var(--color-border-light);
  transition: background var(--transition);
}

.search-result-item:last-child { border-bottom: none; }
.search-result-item:hover,
.search-result-item.active { background: var(--color-primary-light); }

.search-result-item .result-title {
  font-weight: 600;
  font-size: 14px;
}

.search-result-item .result-section {
  font-size: 12px;
  color: var(--color-text-tertiary);
  margin-top: 2px;
}

.search-result-item mark {
  background: #fef08a;
  color: inherit;
  border-radius: 2px;
  padding: 0 1px;
}

.search-no-results {
  padding: 20px 14px;
  text-align: center;
  color: var(--color-text-tertiary);
  font-size: 14px;
}

/* ── Layout ── */
.layout {
  display: flex;
  max-width: 1200px;
  margin: 0 auto;
  min-height: calc(100vh - 56px);
}

/* ── Sidebar ── */
.sidebar {
  width: var(--sidebar-width);
  flex-shrink: 0;
  border-right: 1px solid var(--color-border-light);
  padding: 20px 0;
  position: sticky;
  top: 56px;
  height: calc(100vh - 56px);
  overflow-y: auto;
  scrollbar-width: thin;
}

.sidebar-nav { padding: 0 12px; }

.nav-section { margin-bottom: 4px; }

.nav-section-title {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 12px;
  font-size: 13px;
  font-weight: 600;
  color: var(--color-text);
  cursor: pointer;
  border-radius: var(--radius-sm);
  transition: background var(--transition);
  user-select: none;
}

.nav-section-title:hover { background: var(--color-bg-tertiary); }

.nav-section-link {
  display: block;
  padding: 8px 12px;
  font-size: 13px;
  font-weight: 600;
  color: var(--color-text);
  text-decoration: none;
  border-radius: var(--radius-sm);
  transition: background var(--transition), color var(--transition);
}

.nav-section-link:hover { background: var(--color-bg-tertiary); }

.nav-section-link.active {
  background: var(--color-primary-light);
  color: var(--color-primary);
}

.nav-section-title .chevron {
  width: 16px;
  height: 16px;
  transition: transform var(--transition);
  color: var(--color-text-tertiary);
}

.nav-section.collapsed .chevron { transform: rotate(-90deg); }
.nav-section.collapsed .nav-section-items { display: none; }

.nav-section-items { padding: 2px 0 6px 0; }

.nav-link {
  display: block;
  padding: 6px 12px 6px 24px;
  font-size: 13px;
  color: var(--color-text-secondary);
  text-decoration: none;
  border-radius: var(--radius-sm);
  transition: background var(--transition), color var(--transition);
  line-height: 1.4;
}

.nav-link:hover {
  background: var(--color-bg-tertiary);
  color: var(--color-text);
}

.nav-group-label {
  padding: 10px 12px 4px 24px;
  font-size: 11px;
  font-weight: 700;
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.nav-link.nav-sublink {
  padding-left: 36px;
  font-size: 12.5px;
}

.nav-link.active {
  background: var(--color-primary-light);
  color: var(--color-primary);
  font-weight: 500;
}

/* ── Main content ── */
.main-content {
  flex: 1;
  min-width: 0;
  padding: 32px 40px 64px;
}

/* ── Breadcrumb ── */
.breadcrumb {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 13px;
  color: var(--color-text-tertiary);
  margin-bottom: 24px;
  flex-wrap: wrap;
}

.breadcrumb a {
  color: var(--color-text-tertiary);
  text-decoration: none;
  transition: color var(--transition);
}

.breadcrumb a:hover { color: var(--color-primary); }

.breadcrumb .sep { color: var(--color-border); }

/* ── Homepage ── */
.hero {
  text-align: center;
  padding: 48px 20px 40px;
}

.hero h1 {
  font-size: 32px;
  font-weight: 700;
  margin-bottom: 12px;
  letter-spacing: -0.5px;
}

.hero p {
  font-size: 16px;
  color: var(--color-text-secondary);
  max-width: 520px;
  margin: 0 auto;
}

.home-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 16px;
  padding: 0 20px;
  max-width: 900px;
  margin: 0 auto;
}

.category-card {
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 24px;
  text-decoration: none;
  color: var(--color-text);
  transition: border-color var(--transition), box-shadow var(--transition), transform var(--transition);
}

.category-card:hover {
  border-color: var(--color-primary);
  box-shadow: var(--shadow-md);
  transform: translateY(-1px);
}

.category-card .card-icon {
  width: 36px;
  height: 36px;
  background: var(--color-primary-light);
  border-radius: var(--radius-sm);
  display: flex;
  align-items: center;
  justify-content: center;
  margin-bottom: 14px;
  color: var(--color-primary);
}

.category-card h3 {
  font-size: 15px;
  font-weight: 600;
  margin-bottom: 6px;
}

.category-card p {
  font-size: 13px;
  color: var(--color-text-secondary);
  line-height: 1.5;
}

/* ── Section pages (list of articles) ── */
.section-header {
  margin-bottom: 28px;
}

.section-header h1 {
  font-size: 26px;
  font-weight: 700;
  letter-spacing: -0.3px;
  margin-bottom: 8px;
}

.section-header p {
  color: var(--color-text-secondary);
  font-size: 15px;
}

.article-list {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.article-list-group {
  padding: 16px 0 6px;
  font-size: 11px;
  font-weight: 700;
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.article-list-item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 14px 16px;
  border: 1px solid var(--color-border-light);
  border-radius: var(--radius-sm);
  text-decoration: none;
  color: var(--color-text);
  transition: border-color var(--transition), background var(--transition);
}

.article-list-item:hover {
  border-color: var(--color-border);
  background: var(--color-bg-secondary);
}

.article-list-item .item-icon {
  width: 8px;
  height: 8px;
  background: var(--color-primary-light);
  border: 2px solid var(--color-primary);
  border-radius: 50%;
  flex-shrink: 0;
}

.article-list-item .item-title {
  font-size: 14px;
  font-weight: 500;
}

.article-list-item .arrow {
  margin-left: auto;
  color: var(--color-text-tertiary);
  flex-shrink: 0;
}

/* ── Article content ── */
.article-header {
  margin-bottom: 28px;
  padding-bottom: 20px;
  border-bottom: 1px solid var(--color-border-light);
}

.article-header h1 {
  font-size: 26px;
  font-weight: 700;
  letter-spacing: -0.3px;
  line-height: 1.3;
}

.article-body {
  max-width: 720px;
}

.article-body h2 {
  font-size: 20px;
  font-weight: 700;
  margin: 36px 0 14px;
  padding-bottom: 8px;
  border-bottom: 2px solid var(--color-border-light);
  letter-spacing: -0.2px;
}

.article-body h2:first-child {
  margin-top: 0;
}

.article-body h3 {
  font-size: 16px;
  font-weight: 700;
  margin: 28px 0 8px;
  color: var(--color-text);
  letter-spacing: -0.15px;
}

.article-body p {
  margin-bottom: 14px;
  color: var(--color-text);
}

.article-body ul, .article-body ol {
  margin: 0 0 14px 24px;
}

.article-body li {
  margin-bottom: 6px;
}

.article-body a {
  color: var(--color-primary);
  text-decoration: none;
  font-weight: 500;
  transition: color var(--transition);
}

.article-body a:hover {
  color: var(--color-primary-dark);
  text-decoration: underline;
}

.article-body img {
  max-width: 100%;
  height: auto;
  border-radius: var(--radius-sm);
}

.article-body code {
  background: var(--color-bg-tertiary);
  padding: 2px 6px;
  border-radius: 4px;
  font-size: 13px;
  font-family: 'SF Mono', Menlo, Monaco, 'Courier New', monospace;
}

.article-body pre {
  background: var(--color-text);
  color: #e2e8f0;
  padding: 16px 20px;
  border-radius: var(--radius-sm);
  overflow-x: auto;
  margin-bottom: 16px;
  font-size: 13px;
  line-height: 1.6;
}

.article-body pre code {
  background: none;
  padding: 0;
  color: inherit;
}

.article-body blockquote {
  border-left: 3px solid var(--color-primary);
  padding: 12px 16px;
  margin: 0 0 16px;
  background: var(--color-primary-light);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  color: var(--color-text-secondary);
}

.article-body .callout {
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border-light);
  border-radius: var(--radius-sm);
  padding: 14px 16px;
  margin-bottom: 16px;
  font-size: 14px;
}

.article-body table {
  width: 100%;
  border-collapse: collapse;
  margin-bottom: 16px;
  font-size: 14px;
}

.article-body th, .article-body td {
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid var(--color-border-light);
}

.article-body th {
  font-weight: 600;
  background: var(--color-bg-secondary);
}

/* ── Sub-articles inside a section page ── */
.child-articles {
  margin-top: 24px;
}

.child-articles h2 {
  font-size: 14px;
  font-weight: 600;
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.5px;
  margin-bottom: 12px;
}

/* ── Footer ── */
.site-footer {
  border-top: 1px solid var(--color-border-light);
  padding: 24px;
  text-align: center;
  font-size: 13px;
  color: var(--color-text-tertiary);
}

.site-footer a {
  color: var(--color-primary);
  text-decoration: none;
}

/* ── Mobile overlay ── */
.sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.3);
  z-index: 90;
}

/* ── Responsive ── */
@media (max-width: 768px) {
  .header-search { width: 200px; }
  .header-search .search-shortcut { display: none; }

  .menu-toggle { display: block; }

  .sidebar {
    position: fixed;
    left: -100%;
    top: 56px;
    bottom: 0;
    z-index: 95;
    background: var(--color-bg);
    transition: left var(--transition);
    box-shadow: var(--shadow-lg);
  }

  .sidebar.open { left: 0; }
  .sidebar-overlay.open { display: block; }

  .main-content { padding: 24px 20px 48px; }

  .hero { padding: 32px 0 28px; }
  .hero h1 { font-size: 24px; }

  .home-grid {
    grid-template-columns: 1fr;
    padding: 0;
  }
}

@media (max-width: 480px) {
  .header-search { width: 160px; }
}