/* ═══════════════════════════════════════════
   PianoLab Design System
   Black × Ivory × Gold × Rosewood
   ═══════════════════════════════════════════ */

:root {
  /* ---- palette ---- */
  --piano-black: oklch(12% 0.005 280);
  --ivory: oklch(97% 0.006 85);
  --ivory-warm: oklch(94% 0.012 78);
  --gold: oklch(68% 0.12 80);
  --gold-light: oklch(88% 0.08 80);
  --rosewood: oklch(32% 0.08 28);
  --rosewood-light: oklch(48% 0.06 30);

  /* ---- semantic ---- */
  --bg-primary: var(--ivory);
  --bg-surface: var(--ivory-warm);
  --bg-card: oklch(99% 0.002 85);
  --text-primary: var(--piano-black);
  --text-secondary: oklch(30% 0.005 280);
  --text-tertiary: oklch(55% 0.005 280);
  --action: var(--gold);
  --action-hover: oklch(60% 0.12 80);
  --action-text: oklch(98% 0 0);
  --action-bg: oklch(92% 0.04 80);
  --border-color: oklch(88% 0.008 85);
  --border-light: oklch(92% 0.006 85);

  /* ---- typography ---- */
  --font-body: 'Inter', 'Noto Sans TC', system-ui, -apple-system, sans-serif;
  --font-heading: 'Playfair Display', 'Noto Serif TC', 'Times New Roman', serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', monospace;
  --text-xs: 0.75rem;
  --text-sm: 0.875rem;
  --text-base: 1rem;
  --text-lg: 1.125rem;
  --text-xl: 1.25rem;
  --text-2xl: 1.5rem;
  --reading-max-width: 42rem;

  /* ---- spacing ---- */
  --space-xs: 6px;
  --space-sm: 12px;
  --space-md: 20px;
  --space-lg: 32px;
  --space-xl: 48px;
  --space-2xl: 72px;

  /* ---- radius ---- */
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;

  /* ---- shadows ---- */
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.04);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.06);
}

/* ═══════ RESET ═══════ */
*,*::before,*::after{box-sizing:border-box;margin:0;padding:0}
html{font-size:16px;-webkit-text-size-adjust:100%}
body{font-family:var(--font-body);font-size:var(--text-base);line-height:1.75;color:var(--text-primary);background:var(--bg-primary);-webkit-font-smoothing:antialiased}
img{max-width:100%;height:auto;display:block}
a{color:var(--action);text-decoration:none}
a:hover{color:var(--action-hover)}
ul,ol{padding-left:1.5em}
li{margin-bottom:6px}

/* ═══════ NAVBAR ═══════ */
.navbar{display:flex;align-items:center;justify-content:space-between;padding:0 clamp(12px,3vw,40px);height:56px;background:var(--piano-black);color:#fff;position:sticky;top:0;z-index:100}
.navbar-brand{display:flex;align-items:center;gap:10px;color:#fff;font-family:var(--font-heading);font-size:1.15rem;font-weight:700;letter-spacing:0.03em}
.navbar-brand svg{width:28px;height:28px;flex-shrink:0}
.navbar-nav{display:flex;align-items:center;gap:4px}
.nav-link{color:rgba(255,255,255,0.78);font-size:var(--text-sm);font-weight:600;padding:6px 12px;border-radius:var(--radius-sm);transition:all .15s}
.nav-link:hover,.nav-link.active{color:#fff;background:rgba(255,255,255,0.1)}
.nav-dropdown{position:relative}
.dropdown-btn{background:none;border:none;color:rgba(255,255,255,0.78);font-size:var(--text-sm);font-family:inherit;font-weight:600;cursor:pointer;padding:6px 12px;border-radius:var(--radius-sm);transition:all .15s}
.dropdown-btn:hover,.nav-dropdown.active .dropdown-btn{color:#fff;background:rgba(255,255,255,0.1)}
.dropdown-menu{display:none;position:absolute;top:100%;left:0;background:var(--piano-black);border-radius:var(--radius-sm);min-width:180px;padding:6px 0;box-shadow:0 8px 24px rgba(0,0,0,0.3)}
.nav-dropdown:hover .dropdown-menu,.nav-dropdown.active .dropdown-menu{display:block}
.dropdown-menu a{display:block;padding:8px 16px;color:rgba(255,255,255,0.78);font-size:var(--text-sm);transition:all .12s}
.dropdown-menu a:hover,.dropdown-menu a[aria-current]{color:#fff;background:rgba(255,255,255,0.08)}
.nav-toggle{display:none;background:none;border:none;color:#fff;font-size:1.5rem;cursor:pointer}

/* ═══════ PAGE LAYOUT ═══════ */
.page{min-height:60vh}
.container{width:min(1080px,100%);margin-inline:auto;padding-inline:clamp(16px,4vw,48px)}
.course-breadcrumb{font-size:var(--text-xs);color:var(--text-tertiary);padding:var(--space-sm) 0;display:flex;align-items:center;gap:6px;flex-wrap:wrap}
.course-breadcrumb a{color:var(--text-tertiary)}
.course-breadcrumb .current{color:var(--text-primary);font-weight:700}
.course-breadcrumb .sep{margin:0 2px}
.progress{display:none}

/* ═══════ PAGE HEADER ═══════ */
.page-header{padding:var(--space-xl) 0 var(--space-md);border-bottom:1px solid var(--border-color);margin-bottom:var(--space-lg)}
.page-header-icon{margin-bottom:var(--space-sm)}
.page-header-icon svg{width:32px;height:32px}
.page-header h1{font-family:var(--font-heading);font-size:clamp(1.8rem,4vw,2.6rem);font-weight:800;color:var(--piano-black);letter-spacing:-0.02em;line-height:1.25}
.page-header p{font-size:var(--text-lg);color:var(--text-secondary);max-width:var(--reading-max-width);margin-top:var(--space-sm);line-height:1.7}

/* ═══════ CHAPTER BODY ═══════ */
.chapter-body h3{font-family:var(--font-heading);font-size:1.3rem;color:var(--piano-black);margin:var(--space-lg) 0 var(--space-sm);padding-top:var(--space-sm)}
.chapter-body h4{font-family:var(--font-heading);font-size:1.1rem;color:var(--rosewood);margin:var(--space-md) 0 var(--space-sm)}
.chapter-body p{margin-bottom:var(--space-sm);color:var(--text-secondary);line-height:1.85}
.chapter-body ul,.chapter-body ol{margin-bottom:var(--space-md)}
.chapter-body strong{color:var(--piano-black);font-weight:700}

/* ═══════ TABLES ═══════ */
.data-table{width:100%;border-collapse:collapse;font-size:var(--text-sm);margin:var(--space-md) 0}
.data-table th{background:var(--piano-black);color:#fff;padding:10px 14px;text-align:left;font-weight:700;font-size:var(--text-xs);text-transform:uppercase;letter-spacing:0.04em}
.data-table td{padding:10px 14px;border-bottom:1px solid var(--border-color);color:var(--text-secondary);line-height:1.6}
.data-table tr:hover td{background:var(--bg-surface)}
.ref-table-scroll{overflow-x:auto;-webkit-overflow-scrolling:touch;max-width:var(--reading-max-width);margin:var(--space-md) auto}

/* ═══════ INFO BOX ═══════ */
.info-box{padding:var(--space-md);margin:var(--space-lg) 0;background:var(--action-bg);border-left:4px solid var(--action);border-radius:0 var(--radius-sm) var(--radius-sm) 0;font-size:var(--text-sm);color:var(--text-secondary);line-height:1.75}
.info-box strong{color:var(--rosewood)}

/* ═══════ CHAPTER NAV ═══════ */
.chapter-nav{display:flex;justify-content:space-between;max-width:var(--reading-max-width);margin:var(--space-xl) auto 0;padding-top:var(--space-lg);border-top:1px solid var(--border-color);flex-wrap:wrap;gap:var(--space-sm)}
.chapter-nav a{font-size:var(--text-sm);font-weight:700;color:var(--action);padding:6px 14px;border-radius:var(--radius-sm);transition:all .15s}
.chapter-nav a:hover{color:var(--action-hover);background:var(--action-bg)}

/* ═══════ FOOTER ═══════ */
.footer{padding:var(--space-lg) clamp(16px,4vw,48px);text-align:center;font-size:var(--text-xs);color:var(--text-tertiary);border-top:1px solid var(--border-color);margin-top:var(--space-2xl)}

/* ═══════ RESPONSIVE ═══════ */
@media(max-width:768px){
  .navbar-nav{display:none;position:absolute;top:56px;left:0;right:0;background:var(--piano-black);flex-direction:column;padding:var(--space-sm)}
  .navbar-nav.open{display:flex}
  .nav-toggle{display:block}
  .dropdown-menu{position:static;box-shadow:none;padding-left:var(--space-md)}
}