/* ===== CSS Variables ===== */
:root {
  --my-blue: #3E6D9C;
  --my-blue-dark: #2C5A85;
  --my-blue-light: #5A8DB8;
  --my-orange: #F29111;
  --my-orange-light: #F5A623;
  --bg-primary: #fafbfc;
  --bg-secondary: #f0f2f5;
  --bg-card: #ffffff;
  --bg-code: #1e2a3a;
  --text-primary: #1a2332;
  --text-secondary: #4a5568;
  --text-muted: #8896a6;
  --border-color: #e2e8f0;
  --border-subtle: #edf2f7;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.06), 0 1px 2px rgba(0,0,0,0.04);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.07), 0 2px 4px rgba(0,0,0,0.04);
  --shadow-lg: 0 12px 40px rgba(0,0,0,0.1), 0 4px 12px rgba(0,0,0,0.05);
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-xl: 24px;
  --font-display: 'DM Sans', 'Noto Sans SC', sans-serif;
  --font-body: 'Noto Sans SC', 'DM Sans', sans-serif;
  --font-mono: 'JetBrains Mono', monospace;
  --nav-height: 64px;
  --max-width: 1200px;
  --transition-fast: 0.2s cubic-bezier(0.4, 0, 0.2, 1);
  --transition-smooth: 0.4s cubic-bezier(0.22, 1, 0.36, 1);
}
[data-theme="dark"] {
  --bg-primary: #0f1923;
  --bg-secondary: #162230;
  --bg-card: #1a2d3d;
  --bg-code: #0d1520;
  --text-primary: #e8edf2;
  --text-secondary: #a0b0c0;
  --text-muted: #607080;
  --border-color: #253545;
  --border-subtle: #1e3040;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.2);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.25);
  --shadow-lg: 0 12px 40px rgba(0,0,0,0.35);
  --my-blue-light: #7AADCF;
  --my-orange-light: #F5B84A;
}

/* ===== Reset ===== */
*, *::before, *::after { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: var(--nav-height); }
body {
  font-family: var(--font-body);
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.7;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--my-blue-light); text-decoration: none; transition: color var(--transition-fast); }
a:hover { color: var(--my-blue); }
.container { max-width: var(--max-width); margin: 0 auto; padding: 0 24px; }

/* ===== Navigation ===== */
.nav {
  position: fixed; top: 0; left: 0; right: 0; z-index: 1000;
  height: var(--nav-height);
  background: rgba(250,251,252,0.85);
  backdrop-filter: blur(20px) saturate(180%);
  -webkit-backdrop-filter: blur(20px) saturate(180%);
  border-bottom: 1px solid var(--border-subtle);
  transition: background var(--transition-fast), border-color var(--transition-fast);
}
[data-theme="dark"] .nav { background: rgba(15,25,35,0.88); }
.nav-inner {
  max-width: var(--max-width); margin: 0 auto; padding: 0 24px;
  height: 100%; display: flex; align-items: center; justify-content: space-between;
}
.nav-brand { display: flex; align-items: center; gap: 10px; font-weight: 700; font-size: 16px; color: var(--text-primary); }
.nav-brand-icon {
  width: 32px; height: 32px;
  background: linear-gradient(135deg, var(--my-blue), var(--my-orange));
  border-radius: var(--radius-sm);
  display: flex; align-items: center; justify-content: center;
  font-size: 18px; color: white; font-family: var(--font-mono); font-weight: 700;
  box-shadow: 0 2px 8px rgba(62,109,156,0.3);
}
.nav-brand span { font-family: var(--font-display); letter-spacing: -0.5px; }
.nav-links { display: flex; align-items: center; gap: 6px; }
.nav-links a { padding: 6px 14px; border-radius: var(--radius-sm); font-size: 14px; font-weight: 500; color: var(--text-secondary); transition: all var(--transition-fast); }
.nav-links a:hover { color: var(--text-primary); background: var(--bg-secondary); }
.nav-links a.active { color: var(--my-blue); background: rgba(62,109,156,0.08); }
.nav-right { display: flex; align-items: center; gap: 8px; }
.theme-toggle {
  width: 36px; height: 36px; border-radius: var(--radius-sm);
  border: 1px solid var(--border-color); background: var(--bg-card);
  cursor: pointer; display: flex; align-items: center; justify-content: center;
  font-size: 16px; transition: all var(--transition-fast); color: var(--text-secondary);
}
.theme-toggle:hover { border-color: var(--my-blue); color: var(--my-blue); }
.nav-cta {
  padding: 7px 18px; border-radius: var(--radius-sm); font-size: 13px; font-weight: 600;
  background: var(--my-blue); color: white !important; transition: all var(--transition-fast);
  letter-spacing: 0.2px;
}
.nav-cta:hover { background: var(--my-blue-dark); transform: translateY(-1px); box-shadow: 0 4px 12px rgba(62,109,156,0.3); }
.mobile-toggle { display: none; background: none; border: none; font-size: 22px; cursor: pointer; color: var(--text-primary); padding: 4px; }

/* ===== Back to Top ===== */
.back-to-top {
  position: fixed; bottom: 28px; right: 28px; z-index: 999;
  width: 48px; height: 48px;
  opacity: 0; pointer-events: none;
  transition: all 0.3s ease;
}
.back-to-top.visible { opacity: 1; pointer-events: auto; }
.back-to-top:hover { transform: translateY(-3px); }
.btt-progress {
  position: absolute; top: 0; left: 0; width: 48px; height: 48px;
  transform: rotate(-90deg);
}
.btt-progress circle {
  fill: none; stroke: var(--my-blue); stroke-width: 2.5;
  stroke-dasharray: 125.66; stroke-dashoffset: 125.66;
  stroke-linecap: round; transition: stroke-dashoffset 0.1s linear;
}
.btt-btn {
  position: absolute; top: 4px; left: 4px;
  width: 40px; height: 40px;
  border-radius: 50%; border: 1px solid var(--border-color);
  background: var(--bg-card); color: var(--my-blue);
  font-size: 18px; font-weight: 700; cursor: pointer;
  display: flex; align-items: center; justify-content: center;
  box-shadow: var(--shadow-md); transition: all 0.2s ease;
}
.btt-btn:hover { background: var(--my-blue); color: #fff; }

/* ===== Mobile Responsive (shared) ===== */
@media (max-width: 768px) {
  .nav-links { display: none; }
  .mobile-toggle { display: block; }
  .nav-links.open {
    display: flex; flex-direction: column;
    position: absolute; top: var(--nav-height); left: 0; right: 0;
    background: var(--bg-card); border-bottom: 1px solid var(--border-color);
    padding: 16px 24px; gap: 4px; box-shadow: var(--shadow-md);
  }
}