/* ===== Reset & Base ===== */
*, *::before, *::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --bg: #f5f5f0;
  --bg-card: #ffffff;
  --bg-card-hover: #f0f1ec;
  --text: #0e0f0c;
  --text-dim: #454745;
  --accent: #2e7d32;
  --accent-soft: rgba(159, 232, 112, 0.12);
  --border: rgba(14, 15, 12, 0.08);
  --ring: rgba(14, 15, 12, 0.10);
  --step-active: #9fe870;
  --step-done: #054d28;
  --step-pending: #868685;
  --node-user: #5b5fc7;
  --node-dns: #b45309;
  --node-isp: #047857;
  --node-ixp: #be185d;
  --node-asn: #6d28d9;
  --node-server: #dc2626;
  --accent-glow: rgba(159, 232, 112, 0.35);
  --radius: 16px;
  --radius-lg: 24px;
  --transition: 0.2s ease;
}

html {
  font-size: 16px;
  scroll-behavior: smooth;
}

body {
  font-family: "Inter", "Noto Sans TC", -apple-system, BlinkMacSystemFont, sans-serif;
  font-weight: 500;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  word-break: keep-all;
  overflow-wrap: break-word;
  min-height: 100vh;
  font-feature-settings: "calt" 1;
  -webkit-font-smoothing: antialiased;
}

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

a:hover {
  text-decoration: underline;
}

/* ===== Nav ===== */
.site-nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 1rem 2rem;
  border-bottom: 1px solid var(--border);
  background: rgba(245, 245, 240, 0.85);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  position: sticky;
  top: 0;
  z-index: 100;
}

.site-nav .logo {
  font-size: 1.3rem;
  font-weight: 800;
  color: var(--text);
  letter-spacing: 0.08em;
}

.site-nav .nav-links {
  display: flex;
  gap: 1.5rem;
  list-style: none;
}

.site-nav .nav-links a {
  color: var(--text-dim);
  font-size: 0.9rem;
  font-weight: 500;
  transition: color var(--transition);
}

.site-nav .nav-links a:hover,
.site-nav .nav-links a.active {
  color: var(--text);
  text-decoration: none;
}

/* ===== Layout ===== */
.page-container {
  max-width: 960px;
  margin: 0 auto;
  padding: 2.5rem 1.5rem 4rem;
}

.page-title {
  font-size: 2.4rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  line-height: 1.1;
  margin-bottom: 0.6rem;
}

.page-subtitle {
  color: var(--text-dim);
  font-size: 1.1rem;
  margin-bottom: 2.5rem;
}

/* ===== Step Bar ===== */
.step-bar {
  display: flex;
  gap: 0.5rem;
  margin-bottom: 2rem;
  flex-wrap: wrap;
}

.step-btn {
  padding: 0.5rem 1.1rem;
  border: 1px solid var(--ring);
  border-radius: 999px;
  background: var(--bg-card);
  color: var(--text-dim);
  font-size: 0.85rem;
  font-weight: 500;
  cursor: pointer;
  transition: transform var(--transition), background var(--transition), color var(--transition);
  white-space: nowrap;
}

.step-btn:hover {
  background: var(--bg-card-hover);
  color: var(--text);
  transform: scale(1.04);
}

.step-btn:active {
  transform: scale(0.97);
}

.step-btn.active {
  background: var(--step-active);
  color: #163300;
  border-color: var(--step-active);
  font-weight: 600;
}

.step-btn.done {
  background: transparent;
  border-color: var(--step-done);
  color: var(--step-done);
}

/* ===== SVG Diagram Area ===== */
.diagram-container {
  background: var(--bg-card);
  border: 1px solid var(--ring);
  border-radius: var(--radius-lg);
  padding: 2rem;
  margin-bottom: 2rem;
  overflow-x: auto;
}

.diagram-container svg {
  width: 100%;
  height: auto;
  min-height: 320px;
  display: block;
}

/* SVG node styles */
.node-group {
  cursor: default;
  transform-origin: center center;
  transform-box: fill-box;
}

.node-group:hover {
  opacity: 0.8;
}

.node-group.dimmed {
  opacity: 0.35;
}

.node-group.dimmed:hover {
  opacity: 0.35;
}

.node-group.highlighted .node-box {
  filter: drop-shadow(0 0 12px var(--accent-glow));
}

.node-label {
  fill: var(--text);
  font-size: 16px;
  font-weight: 600;
  font-family: inherit;
  text-anchor: middle;
  paint-order: stroke fill;
  stroke: var(--bg-card);
  stroke-width: 5px;
  stroke-linejoin: round;
}

.node-sublabel {
  fill: var(--text-dim);
  font-size: 12px;
  font-family: inherit;
  text-anchor: middle;
  paint-order: stroke fill;
  stroke: var(--bg-card);
  stroke-width: 4px;
  stroke-linejoin: round;
}

.edge-line {
  stroke: var(--border);
  stroke-width: 2;
  transition: none;
}

.edge-line.active {
  stroke: var(--accent);
  stroke-width: 2.5;
}

.edge-line.dimmed {
  opacity: 0.1;
}

/* ===== Analogy Box (collapsible) ===== */
.analogy-box {
  margin-top: 1rem;
  border: 1px solid var(--ring);
  border-radius: 12px;
  background: var(--accent-soft);
  overflow: hidden;
}

.analogy-box summary {
  padding: 0.65rem 1rem;
  cursor: pointer;
  color: var(--accent);
  font-size: 0.9rem;
  font-weight: 600;
  list-style: none;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  user-select: none;
  transition: background var(--transition);
}

.analogy-box summary:hover {
  background: rgba(159, 232, 112, 0.12);
}

.analogy-box summary::before {
  content: '▶';
  font-size: 0.6rem;
  transition: transform var(--transition);
}

.analogy-box[open] summary::before {
  transform: rotate(90deg);
}

.analogy-box .analogy-content {
  padding: 0 1rem 0.85rem;
  color: var(--text-dim);
  font-size: 0.9rem;
  line-height: 1.75;
}

/* ===== Info Panel ===== */
.info-panel {
  background: var(--bg-card);
  border: 1px solid var(--ring);
  border-radius: var(--radius-lg);
  padding: 2rem;
  min-height: 180px;
  transition: border-color var(--transition);
}

.info-panel.highlight {
  border-color: var(--accent);
}

.info-panel h3 {
  font-size: 1.25rem;
  font-weight: 700;
  margin-bottom: 0.75rem;
  color: var(--accent);
}

.info-panel p {
  color: var(--text-dim);
  line-height: 1.8;
  margin-bottom: 0.75rem;
}

.info-panel .key-point {
  background: var(--accent-soft);
  border-left: 3px solid var(--accent);
  padding: 0.85rem 1.1rem;
  border-radius: 0 12px 12px 0;
  margin-top: 1rem;
  font-size: 0.9rem;
  line-height: 1.7;
}

/* ===== Step Navigation ===== */
.step-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 1.5rem;
}

.step-nav button {
  padding: 0.65rem 1.6rem;
  border: 1px solid var(--ring);
  border-radius: 999px;
  background: var(--bg-card);
  color: var(--text);
  cursor: pointer;
  font-size: 0.9rem;
  font-weight: 600;
  transition: transform var(--transition), background var(--transition);
}

.step-nav button:hover:not(:disabled) {
  background: var(--bg-card-hover);
  transform: scale(1.04);
}

.step-nav button:active:not(:disabled) {
  transform: scale(0.97);
}

.step-nav button:disabled {
  opacity: 0.25;
  cursor: not-allowed;
}

/* ===== Home Page Cards ===== */
.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 1.25rem;
  margin-top: 2rem;
}

.topic-card {
  background: var(--bg-card);
  border: 1px solid var(--ring);
  border-radius: var(--radius-lg);
  padding: 1.75rem;
  transition: transform var(--transition), box-shadow var(--transition);
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.topic-card:hover {
  transform: scale(1.02);
  box-shadow: 0 0 0 1px rgba(46, 125, 50, 0.2), 0 12px 32px rgba(0, 0, 0, 0.08);
}

.topic-card h3 {
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--text);
}

.topic-card p {
  color: var(--text-dim);
  font-size: 0.9rem;
  flex-grow: 1;
  text-wrap: balance;
}

.topic-card .card-tag {
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent);
  border: 1px solid var(--accent);
  padding: 0.2rem 0.65rem;
  border-radius: 999px;
  align-self: flex-start;
}

.topic-card .card-tag.coming-soon {
  color: var(--text-dim);
  border-color: var(--step-pending);
}

/* ===== Footer ===== */
.site-footer {
  text-align: center;
  padding: 2.5rem 2rem;
  color: var(--text-dim);
  font-size: 0.8rem;
  border-top: 1px solid var(--border);
}

/* ===== Responsive ===== */
@media (max-width: 640px) {
  .site-nav {
    padding: 0.75rem 1rem;
  }

  .page-title {
    font-size: 1.75rem;
  }

  .step-bar {
    gap: 0.35rem;
  }

  .step-btn {
    font-size: 0.75rem;
    padding: 0.4rem 0.75rem;
  }

  .diagram-container svg {
    min-height: 220px;
  }

  .diagram-container {
    padding: 1.25rem;
    border-radius: var(--radius);
  }

  .info-panel {
    padding: 1.25rem;
    border-radius: var(--radius);
  }
}