/* ===== CSS Variables & Reset ===== */
:root {
  --bg-primary: #0f172a;
  --bg-secondary: #1e293b;
  --bg-card: #1e293b;
  --bg-card-hover: #334155;
  --text-primary: #f1f5f9;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;
  --accent: #3b82f6;
  --accent-hover: #2563eb;
  --accent-glow: rgba(59, 130, 246, 0.3);
  --border: #334155;
  --gradient-start: #3b82f6;
  --gradient-end: #8b5cf6;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow: 0 4px 24px rgba(0, 0, 0, 0.3);
  --shadow-lg: 0 8px 40px rgba(0, 0, 0, 0.4);
  --transition: 0.3s ease;
  --max-width: 1200px;
  --header-height: 72px;
}

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

html {
  scroll-behavior: smooth;
  scroll-padding-top: var(--header-height);
}

body {
  font-family: 'Segoe UI', -apple-system, BlinkMacSystemFont, sans-serif;
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.7;
  overflow-x: hidden;
}

a {
  color: var(--accent);
  text-decoration: none;
  transition: color var(--transition);
}

a:hover {
  color: var(--accent-hover);
}

ul {
  list-style: none;
}

/* ===== Header & Navigation ===== */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background: rgba(15, 23, 42, 0.85);
  backdrop-filter: blur(16px);
  border-bottom: 1px solid var(--border);
  z-index: 1000;
  transition: box-shadow var(--transition);
}

.header.scrolled {
  box-shadow: 0 2px 20px rgba(0, 0, 0, 0.3);
}

.header-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 24px;
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.logo {
  font-size: 1.4rem;
  font-weight: 800;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  letter-spacing: -0.5px;
}

.nav-list {
  display: flex;
  gap: 8px;
}

.nav-list a {
  color: var(--text-secondary);
  font-size: 0.9rem;
  font-weight: 500;
  padding: 8px 14px;
  border-radius: var(--radius-sm);
  transition: all var(--transition);
}

.nav-list a:hover,
.nav-list a.active {
  color: var(--text-primary);
  background: var(--bg-card);
}

/* Mobile menu button */
.menu-toggle {
  display: none;
  background: none;
  border: none;
  color: var(--text-primary);
  font-size: 1.5rem;
  cursor: pointer;
  padding: 8px;
  border-radius: var(--radius-sm);
  transition: background var(--transition);
}

.menu-toggle:hover {
  background: var(--bg-card);
}

/* ===== Hero Section ===== */
.hero {
  padding: 160px 24px 100px;
  text-align: center;
  position: relative;
  overflow: hidden;
}

.hero::before {
  content: '';
  position: absolute;
  top: -50%;
  left: -50%;
  width: 200%;
  height: 200%;
  background: radial-gradient(circle at 50% 50%, var(--accent-glow) 0%, transparent 50%);
  opacity: 0.4;
  animation: pulse 8s ease-in-out infinite;
}

@keyframes pulse {
  0%, 100% { transform: scale(1); opacity: 0.4; }
  50% { transform: scale(1.1); opacity: 0.6; }
}

.hero-content {
  position: relative;
  max-width: 720px;
  margin: 0 auto;
}

.hero-badge {
  display: inline-block;
  padding: 6px 16px;
  background: rgba(59, 130, 246, 0.15);
  border: 1px solid rgba(59, 130, 246, 0.3);
  border-radius: 50px;
  font-size: 0.85rem;
  color: var(--accent);
  font-weight: 600;
  margin-bottom: 24px;
}

.hero h1 {
  font-size: 3.2rem;
  font-weight: 800;
  line-height: 1.2;
  margin-bottom: 20px;
  letter-spacing: -1px;
}

.hero h1 .gradient-text {
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero p {
  font-size: 1.15rem;
  color: var(--text-secondary);
  max-width: 560px;
  margin: 0 auto 36px;
}

.hero-cta {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 14px 32px;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  color: #fff;
  font-size: 1rem;
  font-weight: 600;
  border: none;
  border-radius: 50px;
  cursor: pointer;
  transition: all var(--transition);
  box-shadow: 0 4px 20px var(--accent-glow);
}

.hero-cta:hover {
  transform: translateY(-2px);
  box-shadow: 0 6px 30px var(--accent-glow);
  color: #fff;
}

/* ===== Section Common ===== */
.section {
  padding: 80px 24px;
  max-width: var(--max-width);
  margin: 0 auto;
}

.section-title {
  font-size: 2rem;
  font-weight: 800;
  text-align: center;
  margin-bottom: 12px;
  letter-spacing: -0.5px;
}

.section-subtitle {
  text-align: center;
  color: var(--text-secondary);
  font-size: 1.05rem;
  margin-bottom: 48px;
}

/* ===== Part Cards Grid ===== */
.cards-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 20px;
}

.part-card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 28px;
  cursor: pointer;
  transition: all var(--transition);
  position: relative;
  overflow: hidden;
}

.part-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--gradient-start), var(--gradient-end));
  transform: scaleX(0);
  transform-origin: left;
  transition: transform var(--transition);
}

.part-card:hover {
  background: var(--bg-card-hover);
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
  border-color: rgba(59, 130, 246, 0.3);
}

.part-card:hover::before {
  transform: scaleX(1);
}

.card-header {
  display: flex;
  align-items: center;
  gap: 14px;
  margin-bottom: 14px;
}

.card-number {
  width: 44px;
  height: 44px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  border-radius: 10px;
  font-weight: 800;
  font-size: 1.1rem;
  color: #fff;
  flex-shrink: 0;
}

.card-title {
  font-size: 1.15rem;
  font-weight: 700;
}

.card-type {
  font-size: 0.85rem;
  color: var(--text-muted);
  margin-top: 2px;
}

.card-desc {
  font-size: 0.92rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

.card-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 16px;
}

.card-tag {
  padding: 4px 10px;
  background: rgba(59, 130, 246, 0.1);
  border: 1px solid rgba(59, 130, 246, 0.2);
  border-radius: 6px;
  font-size: 0.78rem;
  color: var(--accent);
  font-weight: 500;
}

/* ===== Part Detail Sections ===== */
.part-detail {
  padding: 80px 24px;
  border-top: 1px solid var(--border);
}

.part-detail:nth-child(even) {
  background: var(--bg-secondary);
}

.detail-inner {
  max-width: var(--max-width);
  margin: 0 auto;
}

.detail-header {
  display: flex;
  align-items: center;
  gap: 16px;
  margin-bottom: 32px;
}

.detail-number {
  width: 56px;
  height: 56px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: linear-gradient(135deg, var(--gradient-start), var(--gradient-end));
  border-radius: 14px;
  font-weight: 800;
  font-size: 1.3rem;
  color: #fff;
  flex-shrink: 0;
}

.detail-title-group h2 {
  font-size: 1.8rem;
  font-weight: 800;
  letter-spacing: -0.5px;
}

.detail-title-group .detail-type {
  font-size: 0.95rem;
  color: var(--text-muted);
  margin-top: 2px;
}

.detail-body {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 32px;
}

.detail-overview h3,
.detail-strategies h3 {
  font-size: 1.15rem;
  font-weight: 700;
  margin-bottom: 16px;
  color: var(--accent);
}

.detail-overview p {
  color: var(--text-secondary);
  font-size: 0.95rem;
  line-height: 1.8;
  margin-bottom: 16px;
}

.detail-strategies ul {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.detail-strategies li {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  font-size: 0.95rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

.detail-strategies li::before {
  content: '▹';
  color: var(--accent);
  font-weight: 700;
  flex-shrink: 0;
  margin-top: 1px;
}

.tip-box {
  margin-top: 24px;
  padding: 20px;
  background: rgba(59, 130, 246, 0.08);
  border: 1px solid rgba(59, 130, 246, 0.2);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--accent);
}

.tip-box strong {
  display: block;
  color: var(--accent);
  font-size: 0.9rem;
  margin-bottom: 6px;
}

.tip-box p {
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

/* Back to top link */
.back-to-cards {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  margin-top: 32px;
  padding: 10px 20px;
  font-size: 0.88rem;
  color: var(--text-muted);
  border: 1px solid var(--border);
  border-radius: 50px;
  transition: all var(--transition);
}

.back-to-cards:hover {
  color: var(--text-primary);
  border-color: var(--accent);
  background: rgba(59, 130, 246, 0.08);
}

/* ===== Footer ===== */
.footer {
  padding: 48px 24px;
  border-top: 1px solid var(--border);
  text-align: center;
}

.footer-inner {
  max-width: var(--max-width);
  margin: 0 auto;
}

.footer p {
  color: var(--text-muted);
  font-size: 0.88rem;
}

.footer .footer-links {
  display: flex;
  justify-content: center;
  gap: 24px;
  margin-top: 16px;
}

.footer .footer-links a {
  color: var(--text-muted);
  font-size: 0.85rem;
  transition: color var(--transition);
}

.footer .footer-links a:hover {
  color: var(--accent);
}

/* ===== Scroll Animation ===== */
.fade-in {
  opacity: 0;
  transform: translateY(30px);
  transition: opacity 0.6s ease, transform 0.6s ease;
}

.fade-in.visible {
  opacity: 1;
  transform: translateY(0);
}

/* ===== Responsive ===== */
@media (max-width: 768px) {
  :root {
    --header-height: 64px;
  }

  .menu-toggle {
    display: block;
  }

  .nav-list {
    display: none;
    position: absolute;
    top: var(--header-height);
    left: 0;
    right: 0;
    background: var(--bg-secondary);
    border-bottom: 1px solid var(--border);
    flex-direction: column;
    padding: 12px;
  }

  .nav-list.open {
    display: flex;
  }

  .nav-list a {
    padding: 12px 16px;
    font-size: 1rem;
  }

  .hero {
    padding: 130px 20px 70px;
  }

  .hero h1 {
    font-size: 2rem;
  }

  .hero p {
    font-size: 1rem;
  }

  .section {
    padding: 60px 16px;
  }

  .section-title {
    font-size: 1.6rem;
  }

  .cards-grid {
    grid-template-columns: 1fr;
    gap: 16px;
  }

  .part-detail {
    padding: 60px 16px;
  }

  .detail-body {
    grid-template-columns: 1fr;
    gap: 24px;
  }

  .detail-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 12px;
  }

  .detail-title-group h2 {
    font-size: 1.4rem;
  }
}

@media (max-width: 480px) {
  .hero h1 {
    font-size: 1.7rem;
  }

  .cards-grid {
    grid-template-columns: 1fr;
  }

  .card-tags {
    gap: 6px;
  }
}