/* Reset and base */
*, *::before, *::after {
  box-sizing: border-box;
}

:root {
  --color-bg: #fafafa;
  --color-text: #1a1a1a;
  --color-muted: #666;
  --color-border: #e0e0e0;
  --color-accent: #2563eb;
  --color-physics: #7c3aed;
  --color-ai: #059669;
  --color-linkedin: #0077b5;
  --font-sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-mono: "SF Mono", Consolas, monospace;
  --max-width: 48rem;
}

body {
  font-family: var(--font-sans);
  font-size: 1rem;
  line-height: 1.7;
  color: var(--color-text);
  background: var(--color-bg);
  margin: 0;
  padding: 0;
}

/* Navigation */
.site-nav {
  border-bottom: 1px solid var(--color-border);
  padding: 1rem 0;
}

.nav-container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 1.5rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.site-title {
  font-weight: 600;
  font-size: 1.1rem;
  color: var(--color-text);
  text-decoration: none;
}

.nav-links {
  display: flex;
  gap: 1.5rem;
}

.nav-links a {
  color: var(--color-muted);
  text-decoration: none;
  font-size: 0.9rem;
}

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

/* Main content */
.content {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 2rem 1.5rem;
}

/* Landing page */
.landing-header {
  text-align: center;
  margin-bottom: 3rem;
}

.landing-header h1 {
  font-size: 2.5rem;
  margin-bottom: 0.5rem;
}

.tagline {
  color: var(--color-muted);
  font-size: 1.1rem;
}

.section-cards {
  display: grid;
  gap: 1.5rem;
}

@media (min-width: 640px) {
  .section-cards {
    grid-template-columns: repeat(3, 1fr);
  }
}

.section-card {
  display: block;
  padding: 1.5rem;
  border: 1px solid var(--color-border);
  border-radius: 8px;
  text-decoration: none;
  color: inherit;
  transition: border-color 0.2s, box-shadow 0.2s;
}

.section-card:hover {
  border-color: var(--color-accent);
  box-shadow: 0 4px 12px rgba(0,0,0,0.08);
}

.card-icon {
  font-size: 2rem;
  margin-bottom: 0.75rem;
}

.section-card h2 {
  font-size: 1.1rem;
  margin: 0 0 0.5rem 0;
}

.section-card p {
  font-size: 0.9rem;
  color: var(--color-muted);
  margin: 0 0 1rem 0;
}

.card-meta {
  font-size: 0.8rem;
  color: var(--color-muted);
}

.section-card.physics { border-left: 3px solid var(--color-physics); }
.section-card.ai { border-left: 3px solid var(--color-ai); }
.section-card.linkedin { border-left: 3px solid var(--color-linkedin); }

/* Section index pages */
.section-header {
  margin-bottom: 2rem;
}

.section-header h1 {
  margin-bottom: 0.25rem;
}

.section-subtitle {
  font-size: 1.1rem;
  color: var(--color-muted);
  margin-bottom: 0.5rem;
}

.section-header .author {
  font-size: 0.95rem;
  color: var(--color-muted);
  margin-top: 1rem;
}

.section-description {
  color: var(--color-muted);
  font-size: 1.05rem;
}

/* Chapter list (physics TOC) */
.chapter-list {
  list-style: none;
  padding: 0;
}

.chapter-list li {
  padding: 1rem 0;
  border-bottom: 1px solid var(--color-border);
}

.chapter-list a {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  text-decoration: none;
  color: var(--color-text);
}

.chapter-list a:hover .chapter-title {
  color: var(--color-accent);
}

.chapter-revision {
  font-size: 0.8rem;
  color: var(--color-muted);
}

.chapter-summary {
  font-size: 0.9rem;
  color: var(--color-muted);
  margin: 0.5rem 0 0 0;
}

/* Post list (AI articles) */
.post-preview {
  padding: 1.5rem 0;
  border-bottom: 1px solid var(--color-border);
}

.post-preview h2 {
  font-size: 1.2rem;
  margin: 0 0 0.5rem 0;
}

.post-preview h2 a {
  color: inherit;
  text-decoration: none;
}

.post-preview h2 a:hover {
  color: var(--color-accent);
}

.post-meta {
  font-size: 0.85rem;
  color: var(--color-muted);
  margin-bottom: 0.5rem;
}

.tag {
  background: var(--color-border);
  padding: 0.1rem 0.5rem;
  border-radius: 3px;
  font-size: 0.75rem;
  margin-left: 0.5rem;
}

.excerpt {
  color: var(--color-muted);
  margin: 0;
}

/* Link list (LinkedIn) */
.link-preview {
  padding: 1rem 0;
  border-bottom: 1px solid var(--color-border);
}

.link-preview h2 {
  font-size: 1.1rem;
  margin: 0 0 0.25rem 0;
}

.link-preview h2 a {
  color: inherit;
  text-decoration: none;
}

.link-preview h2 a:hover {
  color: var(--color-linkedin);
}

.external-icon {
  font-size: 0.8em;
  margin-left: 0.25rem;
}

/* Article pages */
.chapter-header, .post-header, .link-header {
  margin-bottom: 2rem;
}

.chapter-meta {
  font-size: 0.85rem;
  color: var(--color-muted);
  margin-bottom: 0.5rem;
}

.section-label {
  color: var(--color-physics);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.chapter-number {
  margin-left: 1rem;
}

.chapter-summary-header {
  font-size: 1.05rem;
  color: var(--color-muted);
  font-style: italic;
  margin-top: 0.5rem;
}

.revision {
  font-size: 0.9rem;
  color: var(--color-muted);
}

/* Chapter navigation */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--color-border);
  font-size: 0.9rem;
}

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

.nav-toc {
  color: var(--color-muted);
}

/* Content styling */
.chapter-content, .post-content {
  line-height: 1.8;
}

.chapter-content h2, .post-content h2 {
  margin-top: 2rem;
}

.chapter-content code, .post-content code {
  font-family: var(--font-mono);
  font-size: 0.9em;
  background: #f0f0f0;
  padding: 0.1em 0.3em;
  border-radius: 3px;
}

.chapter-content pre, .post-content pre {
  background: #1a1a1a;
  color: #f0f0f0;
  padding: 1rem;
  border-radius: 6px;
  overflow-x: auto;
}

.chapter-content pre code, .post-content pre code {
  background: none;
  padding: 0;
}

/* External link button */
.link-action {
  margin: 2rem 0;
}

.external-link {
  display: inline-block;
  background: var(--color-linkedin);
  color: white;
  padding: 0.75rem 1.5rem;
  border-radius: 6px;
  text-decoration: none;
  font-weight: 500;
}

.external-link:hover {
  background: #005fa3;
}

/* Footer */
.site-footer {
  border-top: 1px solid var(--color-border);
  padding: 2rem 0;
  margin-top: 3rem;
}

.footer-container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 1.5rem;
  text-align: center;
  color: var(--color-muted);
  font-size: 0.9rem;
}

.post-footer, .link-footer {
  margin-top: 2rem;
  padding-top: 1rem;
  border-top: 1px solid var(--color-border);
}

.post-footer a, .link-footer a {
  color: var(--color-accent);
  text-decoration: none;
}