/* ====== DESIGN TOKENS ====== */
:root{
  /* Surfaces */
  --bg:#f8fafc;
  --surface:#ffffff;
  --surface-hover:#f1f5f9;
  --surface-raised:#ffffff;

  /* Sidebar */
  --sidebar-bg:#0f1729;
  --sidebar-surface:#1a2332;
  --sidebar-text:#94a3b8;
  --sidebar-text-bright:#e2e8f0;
  --sidebar-active:#38bdf8;
  --sidebar-active-bg:rgba(56,189,248,0.08);
  --sidebar-divider:rgba(255,255,255,0.06);

  /* Text */
  --text:#0f172a;
  --text-secondary:#475569;
  --text-tertiary:#94a3b8;

  /* Accent */
  --accent:#3b82f6;
  --accent-soft:#dbeafe;
  --accent-glow:rgba(59,130,246,0.25);

  /* Semantic */
  --green:#10b981;--green-soft:#d1fae5;
  --orange:#f59e0b;--orange-soft:#fef3c7;
  --red:#ef4444;--red-soft:#fee2e2;
  --purple:#8b5cf6;--purple-soft:#ede9fe;

  /* Code */
  --code-bg:#faf9f5;
  --code-border:#e8e5dc;
  --code-text:#24292e;
  --code-kw:#0550ae;
  --code-st:#0a3069;
  --code-no:#0550ae;
  --code-cm:#6e7781;

  /* Borders & Shadows */
  --border:#e2e8f0;
  --border-light:#f1f5f9;
  --radius-sm:8px;
  --radius:12px;
  --radius-lg:16px;
  --shadow-xs:0 1px 2px rgba(0,0,0,0.04);
  --shadow-sm:0 1px 3px rgba(0,0,0,0.06),0 1px 2px rgba(0,0,0,0.04);
  --shadow:0 4px 6px -1px rgba(0,0,0,0.05),0 2px 4px -2px rgba(0,0,0,0.04);
  --shadow-md:0 10px 15px -3px rgba(0,0,0,0.06),0 4px 6px -4px rgba(0,0,0,0.04);
  --shadow-lg:0 20px 25px -5px rgba(0,0,0,0.08),0 8px 10px -6px rgba(0,0,0,0.04);

  /* Nav */
  --nav-bg:rgba(15,23,41,0.92);
  --nav-blur:12px;

  /* Transition */
  --ease-out:cubic-bezier(0.16,1,0.3,1);
  --ease-in-out:cubic-bezier(0.65,0,0.35,1);
  --dur-fast:0.15s;
  --dur:0.25s;
  --dur-slow:0.4s;
}

/* ====== RESET & BASE ====== */
*,*::before,*::after{margin:0;padding:0;box-sizing:border-box}
html{-webkit-font-smoothing:antialiased;-moz-osx-font-smoothing:grayscale;text-rendering:optimizeLegibility}
body{
  font-family:'Inter',system-ui,-apple-system,'PingFang SC','Microsoft YaHei','Noto Sans SC',sans-serif;
  background:var(--bg);
  color:var(--text);
  min-height:100vh;
  line-height:1.75;
  font-size:15px;
}

/* ====== SCROLLBAR ====== */
::-webkit-scrollbar{width:6px;height:6px}
::-webkit-scrollbar-track{background:transparent}
::-webkit-scrollbar-thumb{background:#cbd5e1;border-radius:3px}
::-webkit-scrollbar-thumb:hover{background:#94a3b8}
.sidebar::-webkit-scrollbar-thumb{background:rgba(255,255,255,0.15)}
.sidebar::-webkit-scrollbar-thumb:hover{background:rgba(255,255,255,0.25)}

/* ====== TOP NAV ====== */
.top-nav{
  position:fixed;top:0;left:0;right:0;z-index:1000;
  background:var(--nav-bg);
  backdrop-filter:blur(var(--nav-blur));
  -webkit-backdrop-filter:blur(var(--nav-blur));
  display:flex;align-items:center;
  box-shadow:0 1px 2px rgba(0,0,0,0.2);
  height:56px;
}
.top-nav .brand{
  height:26px;width:auto;display:block;margin:0 20px;
}
.top-nav .tabs{display:flex;overflow-x:auto;height:100%;margin:0 auto;transform:translateX(-60px)}
.top-nav .tab{
  padding:0 18px;color:rgba(255,255,255,0.55);cursor:pointer;
  font-size:0.82rem;font-weight:500;white-space:nowrap;
  transition:color var(--dur-fast) var(--ease-out),
             background var(--dur-fast) var(--ease-out);
  text-decoration:none;user-select:none;
  height:100%;display:flex;align-items:center;
  position:relative;
}
.top-nav .tab::after{
  content:'';position:absolute;bottom:0;left:18px;right:18px;
  height:2.5px;background:var(--sidebar-active);
  border-radius:2px 2px 0 0;
  transform:scaleX(0);transition:transform var(--dur) var(--ease-out);
}
.top-nav .tab:hover{color:rgba(255,255,255,0.85);background:rgba(255,255,255,0.04)}
.top-nav .tab.active{color:#fff;background:rgba(56,189,248,0.1)}
.top-nav .tab.active::after{transform:scaleX(1)}

/* ====== LAYOUT ====== */
.app-container{
  margin-top:56px;
  min-height:calc(100vh - 56px);
}

/* ====== SIDEBAR ====== */
.sidebar{
  width:260px;min-width:260px;
  background:var(--sidebar-bg);
  color:var(--sidebar-text);
  position:fixed;top:56px;left:0;bottom:0;
  overflow-y:auto;overflow-x:hidden;
  box-shadow:1px 0 0 rgba(255,255,255,0.04),4px 0 20px rgba(0,0,0,0.25);
  z-index:99;display:flex;flex-direction:column;
}
.sidebar-header{
  padding:18px 16px 14px;
  border-bottom:1px solid var(--sidebar-divider);
  text-align:center;
}
.sidebar-header .level-badge{
  display:inline-block;padding:5px 14px;border-radius:20px;
  font-size:0.72rem;font-weight:700;color:#fff;
  letter-spacing:0.04em;
}
.sidebar-header .logo{
  font-size:0.95rem;font-weight:700;color:#fff;
  margin-top:6px;letter-spacing:-0.01em;
}
.sidebar-nav{flex:1;padding:6px 0}
.sidebar-nav a{
  display:flex;align-items:center;gap:8px;
  padding:10px 18px;cursor:pointer;
  border-left:3px solid transparent;
  font-size:0.84rem;color:var(--sidebar-text);
  transition:all var(--dur-fast) var(--ease-out);
  text-decoration:none;
  margin:2px 0;
}
.sidebar-nav a:hover{
  background:var(--sidebar-surface);
  color:var(--sidebar-text-bright);
}
.sidebar-nav a.active{
  background:var(--sidebar-active-bg);
  border-left-color:var(--sidebar-active);
  color:#fff;font-weight:600;
  box-shadow:inset 0 1px 0 rgba(255,255,255,0.03),0 0 12px rgba(56,189,248,0.08);
}
.sidebar-footer{
  padding:12px 16px;
  border-top:1px solid var(--sidebar-divider);
  font-size:0.6rem;color:rgba(255,255,255,0.25);
  text-align:center;line-height:1.6;
}

/* ====== MAIN CONTENT ====== */
.main-content{
  margin-left:max(260px, calc((100% - 260px - 1000px) / 2 + 260px));
  margin-right:max(0px, calc((100% - 260px - 1000px) / 2));
  padding:28px 48px 80px;
  max-width:1000px;
}
.main-content h1{
  font-size:1.75rem;font-weight:700;color:var(--text);
  margin-bottom:4px;letter-spacing:-0.02em;
  line-height:1.3;
}
.lesson-sub{
  font-size:0.88rem;color:var(--text-tertiary);
  margin-bottom:24px;
}

/* ====== CARD ====== */
.card{
  background:var(--surface);
  border-radius:var(--radius);
  padding:22px 28px;margin-bottom:18px;
  box-shadow:var(--shadow-sm);
  border:1px solid var(--border);
  transition:box-shadow var(--dur) var(--ease-out),
             transform var(--dur) var(--ease-out);
}
.card:hover{
  box-shadow:var(--shadow);
}
.card h3{
  font-size:1.1rem;margin-bottom:10px;color:var(--text);
  font-weight:650;letter-spacing:-0.01em;
  display:flex;align-items:center;gap:8px;
}
.card h4{
  font-size:0.95rem;margin:16px 0 6px;color:#334155;
  font-weight:600;
}
.card p,.card li{font-size:0.9rem;color:var(--text-secondary);margin-bottom:6px;line-height:1.7}
.card ul,.card ol{padding-left:22px}
.card ul li::marker{color:var(--accent)}

/* ====== CODE BLOCK ====== */
.code-block{
  background:var(--code-bg);
  color:var(--code-text);
  padding:16px 20px;border-radius:var(--radius-sm);
  font-family:'JetBrains Mono','Fira Code','Consolas','Monaco','Courier New',monospace;
  font-size:0.82rem;line-height:1.65;
  margin:12px 0;overflow-x:auto;
  border:1px solid var(--code-border);
  white-space:pre;
  position:relative;
}
.code-block::before{
  content:'C++';
  position:absolute;top:0;right:0;
  padding:3px 10px;
  font-size:0.6rem;font-weight:600;color:var(--text-tertiary);
  background:var(--border-light);
  border-radius:0 var(--radius-sm) 0 var(--radius-sm);
  font-family:inherit;
  letter-spacing:0.05em;
  text-transform:uppercase;
}
.kw{color:var(--code-kw);font-weight:600}
.st{color:var(--code-st)}
.no{color:var(--code-no)}
.cm{color:var(--code-cm);font-style:italic}

/* ====== TABLE ====== */
.tbl{
  width:100%;border-collapse:separate;border-spacing:0;
  margin:12px 0;font-size:0.85rem;
  border-radius:var(--radius-sm);
  overflow:hidden;
  border:1px solid var(--border);
}
.tbl thead th{
  background:linear-gradient(180deg,#f8fafc 0%,#f1f5f9 100%);
  padding:10px 14px;text-align:left;font-weight:600;
  font-size:0.78rem;color:var(--text-secondary);
  border-bottom:1px solid var(--border);
  letter-spacing:0.02em;
}
.tbl td{
  padding:9px 14px;
  border-bottom:1px solid var(--border-light);
  color:var(--text-secondary);
}
.tbl tbody tr:last-child td{border-bottom:none}
.tbl tbody tr:nth-child(even){background:#f8fafc}
.tbl .hl-row{background:#f0f9ff !important}
.tbl .hl-row td{color:var(--text)}

/* ====== CALLOUTS ====== */
.hl{
  padding:12px 18px;border-radius:var(--radius-sm);
  margin:12px 0;font-size:0.85rem;line-height:1.65;
  border-left:4px solid;
  position:relative;
}
.hl.info{background:var(--accent-soft);border-color:var(--accent);color:#1e40af}
.hl.success{background:var(--green-soft);border-color:var(--green);color:#065f46}
.hl.warning{background:var(--orange-soft);border-color:var(--orange);color:#92400e}
.hl.danger{background:var(--red-soft);border-color:var(--red);color:#991b1b}
.hl.tip{background:var(--purple-soft);border-color:var(--purple);color:#5b21b6}

/* ====== STORY BOX ====== */
.story-box{
  background:linear-gradient(135deg,#fffbeb 0%,#fef3c7 50%,#fde68a 100%);
  border:1px solid #fcd34d;
  border-radius:var(--radius);
  padding:16px 20px;margin:10px 0;
  font-size:0.88rem;color:#78350f;line-height:1.7;
  position:relative;
}
.story-box::before{
  content:'💡';
  position:absolute;top:-12px;left:16px;
  font-size:1.3rem;
  background:#fff;border-radius:50%;
  width:28px;height:28px;
  display:flex;align-items:center;justify-content:center;
  box-shadow:0 1px 3px rgba(0,0,0,0.1);
}

/* ====== QUESTION BOX ====== */
.question-box{
  background:#f8fafc;
  border:1px dashed #cbd5e1;
  border-radius:var(--radius-sm);
  padding:14px 18px;margin:8px 0;
}

/* ====== LESSON HEADER ====== */
.lesson-title{
  font-size:1.75rem;font-weight:700;margin-bottom:4px;
  color:var(--text);letter-spacing:-0.02em;
  padding-bottom:10px;
  border-bottom:2px solid var(--border);
}

/* ====== STEP LIST ====== */
.step-list{list-style:none;padding:0}
.step-list li{display:flex;align-items:flex-start;gap:12px;margin-bottom:14px;font-size:0.88rem}
.step-num{
  display:flex;align-items:center;justify-content:center;
  width:30px;height:30px;min-width:30px;
  background:linear-gradient(135deg,var(--accent),#6366f1);
  color:#fff;border-radius:50%;
  font-size:0.78rem;font-weight:700;
  box-shadow:0 2px 6px var(--accent-glow);
}

/* ====== SECTION DIVIDER ====== */
.section-divider{
  text-align:center;margin:32px 0 20px;
  font-size:1.2rem;font-weight:700;color:var(--text);
  letter-spacing:-0.01em;
}
.section-divider::after{
  content:'';display:block;
  width:48px;height:3px;
  background:linear-gradient(90deg,var(--accent),#6366f1);
  margin:10px auto 0;border-radius:2px;
}

/* ====== MIND MAP ====== */
.mind-map{display:flex;flex-wrap:wrap;gap:10px;justify-content:center;margin:16px 0}
.mind-node{
  background:var(--accent-soft);
  border:1.5px solid var(--accent);
  border-radius:var(--radius);
  padding:10px 18px;font-weight:600;font-size:0.85rem;
  color:#1e40af;text-align:center;min-width:90px;
  transition:transform var(--dur-fast) var(--ease-out),
             box-shadow var(--dur-fast) var(--ease-out);
  cursor:default;
}
.mind-node:hover{
  transform:translateY(-2px);
  box-shadow:0 4px 12px var(--accent-glow);
}

/* ====== QUICK CARD ====== */
.quick-card{
  display:inline-block;
  background:#ecfdf5;border:1px solid #a7f3d0;
  border-radius:var(--radius-sm);
  padding:7px 16px;margin:3px;
  font-size:0.83rem;color:#065f46;font-weight:500;
  transition:all var(--dur-fast) var(--ease-out);
}
.quick-card:hover{
  background:#d1fae5;
  transform:translateY(-1px);
}

/* ====== PLACEHOLDER ====== */
.placeholder-card{text-align:center;padding:80px 20px}
.placeholder-card .icon{font-size:3.5rem;margin-bottom:20px;opacity:0.6}
.placeholder-card h2{font-size:1.4rem;color:var(--text);margin-bottom:10px}
.placeholder-card p{color:var(--text-tertiary);font-size:0.95rem;max-width:400px;margin:0 auto}

/* ====== NOTE ====== */
.note{font-size:0.8rem;color:var(--text-tertiary);margin-top:4px;font-style:italic}

/* ====== ANIMATIONS ====== */
@keyframes fadeInUp{
  from{opacity:0;transform:translateY(12px)}
  to{opacity:1;transform:translateY(0)}
}
.card{animation:fadeInUp var(--dur-slow) var(--ease-out) both}
.card:nth-child(1){animation-delay:0s}
.card:nth-child(2){animation-delay:0.05s}
.card:nth-child(3){animation-delay:0.1s}
.card:nth-child(4){animation-delay:0.15s}
.card:nth-child(5){animation-delay:0.2s}
.card:nth-child(6){animation-delay:0.25s}

/* ====== SELECTION ====== */
::selection{background:rgba(59,130,246,0.18);color:var(--text)}

/* ====== FOCUS ====== */
a:focus-visible,button:focus-visible{
  outline:2px solid var(--accent);
  outline-offset:2px;
  border-radius:4px;
}

/* ====== TAB INDEX INDICATOR ====== */
.top-nav .tab[data-lvl]::before{
  content:attr(data-lvl);
  display:inline-flex;align-items:center;justify-content:center;
  width:20px;height:20px;border-radius:50%;
  font-size:0.65rem;font-weight:700;
  margin-right:6px;
  background:rgba(255,255,255,0.08);
  color:rgba(255,255,255,0.5);
  transition:all var(--dur-fast) var(--ease-out);
  flex-shrink:0;
}
.top-nav .tab.active::before{
  background:var(--sidebar-active);
  color:#fff;
  box-shadow:0 0 12px var(--accent-glow);
}

/* ====== HAMBURGER ====== */
.hamburger{display:none;background:none;border:none;color:#fff;font-size:1.3rem;cursor:pointer;padding:10px 16px;margin-left:auto;transition:color var(--dur-fast)}
.hamburger:hover{color:var(--sidebar-active)}
.sidebar-overlay{display:none;position:fixed;top:0;left:0;right:0;bottom:0;background:rgba(0,0,0,0.5);z-index:98;backdrop-filter:blur(2px);-webkit-backdrop-filter:blur(2px)}

/* ====== RESPONSIVE: TABLET ====== */
@media(max-width:1024px){
  .top-nav .tab{padding:0 13px;font-size:0.78rem}
  .top-nav .tab[data-lvl]::before{width:18px;height:18px;font-size:0.6rem}
  .sidebar{width:220px;min-width:220px}
  .main-content{margin-left:220px;margin-right:0;padding:22px 28px 60px;max-width:100%}
  .card{padding:18px 20px}
  .card h3{font-size:1.04rem}
  .card p,.card li{font-size:0.86rem}
}
@media(max-width:768px){
  .hamburger{display:block}
  .top-nav .brand{height:20px;width:auto;margin:0 12px}
  .top-nav .tabs{overflow-x:auto;-webkit-overflow-scrolling:touch;scrollbar-width:none;margin:0;transform:none}
  .top-nav .tabs::-webkit-scrollbar{display:none}
  .top-nav .tab{padding:0 12px;font-size:0.76rem}
  .top-nav .tab[data-lvl]::before{width:16px;height:16px;font-size:0.58rem;margin-right:4px}
  .app-container{margin-top:56px}
  .sidebar{position:fixed;top:56px;left:-270px;bottom:0;width:270px;min-width:270px;z-index:99;transition:left 0.35s var(--ease-out)}
  .sidebar.open{left:0}
  .sidebar-overlay.active{display:block}
  .main-content{margin-left:0;margin-right:0;padding:18px 14px 60px;max-width:100%}
  .card{padding:16px 14px;margin-bottom:14px;border-radius:var(--radius-sm)}
  .card h3{font-size:0.98rem}
  .card h4{font-size:0.9rem}
  .card p,.card li{font-size:0.83rem}
  .main-content h1{font-size:1.4rem}
  .lesson-sub{font-size:0.82rem}
  .code-block{font-size:0.74rem;padding:12px 14px;overflow-x:auto;-webkit-overflow-scrolling:touch;border-radius:var(--radius-sm)}
  .code-block::before{font-size:0.55rem;padding:2px 8px}
  .tbl{font-size:0.74rem;display:block;overflow-x:auto;-webkit-overflow-scrolling:touch}
  .hl{padding:10px 14px;font-size:0.82rem}
  .story-box{font-size:0.83rem;padding:14px 16px}
  .story-box::before{top:-10px;left:12px;width:24px;height:24px;font-size:1.1rem}
  .mind-map{gap:8px}
  .mind-node{padding:8px 12px;font-size:0.78rem;min-width:70px}
  .section-divider{font-size:1.05rem;margin:24px 0 16px}
  .sidebar-header{padding:14px 12px 10px}
  .sidebar-nav a{padding:8px 14px;font-size:0.78rem}
}
@media(max-width:480px){
  .top-nav .brand{height:18px;width:auto;margin:0 8px}
  .top-nav .tab{padding:0 10px;font-size:0.7rem}
  .top-nav .tab[data-lvl]::before{width:14px;height:14px;font-size:0.55rem}
  .sidebar{width:250px;min-width:250px;left:-250px}
  .main-content{padding:14px 10px 50px}
  .card{padding:14px 12px;border-radius:8px}
  .card h3{font-size:0.92rem}
  .card p,.card li{font-size:0.8rem}
  .main-content h1{font-size:1.25rem}
  .code-block{font-size:0.68rem;padding:10px 12px}
  .hl{padding:8px 12px;font-size:0.78rem}
}