/* ===== CSS Variables ===== */
:root {
  --primary-color: #1a73e8;
  --primary-dark: #1557b0;
  --secondary-color: #34a853;
  --text-color: #202124;
  --text-light: #5f6368;
  --bg-color: #ffffff;
  --bg-secondary: #f8f9fa;
  --border-color: #dadce0;
  --sidebar-width: 280px;
  --header-height: 64px;
}

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

html {
  scroll-behavior: smooth;
}

body {
  font-family: 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  font-size: 16px;
  line-height: 1.7;
  color: var(--text-color);
  background: var(--bg-color);
}

a {
  color: var(--primary-color);
  text-decoration: none;
  transition: color 0.2s;
}

a:hover {
  color: var(--primary-dark);
  text-decoration: underline;
}

/* ===== Header ===== */
.header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background: var(--bg-color);
  border-bottom: 1px solid var(--border-color);
  display: flex;
  align-items: center;
  padding: 0 24px;
  z-index: 1000;
  box-shadow: 0 1px 3px rgba(0,0,0,0.1);
}

.logo {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--primary-color);
  display: flex;
  align-items: center;
  gap: 10px;
}

.logo-icon {
  width: 32px;
  height: 32px;
  background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  color: white;
  font-weight: bold;
}

.nav-main {
  margin-left: auto;
  display: flex;
  gap: 24px;
}

.nav-main a {
  color: var(--text-light);
  font-weight: 500;
  padding: 8px 0;
  border-bottom: 2px solid transparent;
}

.nav-main a:hover,
.nav-main a.active {
  color: var(--primary-color);
  text-decoration: none;
  border-bottom-color: var(--primary-color);
}

/* ===== Layout ===== */
.main-content {
  margin-top: var(--header-height);
  min-height: calc(100vh - var(--header-height));
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px;
}

/* ===== Home Page ===== */
.hero {
  background: linear-gradient(135deg, var(--primary-color) 0%, #4285f4 50%, var(--secondary-color) 100%);
  color: white;
  padding: 80px 24px;
  text-align: center;
}

.hero h1 {
  font-size: 3rem;
  margin-bottom: 16px;
  font-weight: 700;
}

.hero p {
  font-size: 1.25rem;
  opacity: 0.9;
  max-width: 600px;
  margin: 0 auto;
}

.topics-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(350px, 1fr));
  gap: 24px;
  padding: 48px 0;
}

.topic-card {
  background: var(--bg-color);
  border: 1px solid var(--border-color);
  border-radius: 12px;
  padding: 24px;
  transition: all 0.3s;
  display: flex;
  flex-direction: column;
}

.topic-card:hover {
  border-color: var(--primary-color);
  box-shadow: 0 4px 20px rgba(26, 115, 232, 0.15);
  transform: translateY(-2px);
}

.topic-card h2 {
  font-size: 1.25rem;
  margin-bottom: 8px;
  color: var(--text-color);
}

.topic-card .meta {
  font-size: 0.875rem;
  color: var(--text-light);
  margin-bottom: 12px;
}

.topic-card .description {
  color: var(--text-light);
  flex-grow: 1;
  margin-bottom: 16px;
  display: -webkit-box;
  -webkit-line-clamp: 3;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.topic-card .read-more {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  color: var(--primary-color);
  font-weight: 500;
}

.topic-card .read-more:hover {
  text-decoration: none;
  gap: 10px;
}

/* ===== Article Page Layout ===== */
.article-layout {
  display: flex;
  gap: 0;
}

/* Sidebar */
.sidebar {
  width: var(--sidebar-width);
  position: fixed;
  left: 0;
  top: var(--header-height);
  bottom: 0;
  background: var(--bg-secondary);
  border-right: 1px solid var(--border-color);
  overflow-y: auto;
  padding: 24px 0;
}

.sidebar-header {
  padding: 0 20px 16px;
  border-bottom: 1px solid var(--border-color);
  margin-bottom: 16px;
}

.sidebar-header h3 {
  font-size: 0.875rem;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--text-light);
  margin-bottom: 8px;
}

.sidebar-header .article-title {
  font-size: 1rem;
  font-weight: 600;
  color: var(--text-color);
}

.toc-list {
  list-style: none;
}

.toc-list li {
  margin: 0;
}

.toc-list a {
  display: block;
  padding: 10px 20px;
  color: var(--text-light);
  font-size: 0.9rem;
  border-left: 3px solid transparent;
  transition: all 0.2s;
}

.toc-list a:hover {
  background: rgba(26, 115, 232, 0.08);
  color: var(--primary-color);
  text-decoration: none;
}

.toc-list a.active {
  background: rgba(26, 115, 232, 0.1);
  color: var(--primary-color);
  border-left-color: var(--primary-color);
  font-weight: 500;
}

.toc-list .toc-sub {
  padding-left: 36px;
  font-size: 0.85rem;
}

/* Article Content */
.article-content {
  flex: 1;
  margin-left: var(--sidebar-width);
  padding: 40px 60px;
  max-width: 900px;
}

.article-header {
  margin-bottom: 32px;
  padding-bottom: 24px;
  border-bottom: 1px solid var(--border-color);
}

.article-header h1 {
  font-size: 2.5rem;
  line-height: 1.2;
  margin-bottom: 16px;
  color: var(--text-color);
}

.article-meta {
  display: flex;
  gap: 24px;
  color: var(--text-light);
  font-size: 0.9rem;
}

.article-meta span {
  display: flex;
  align-items: center;
  gap: 6px;
}

/* Article Sections */
.article-section {
  margin-bottom: 40px;
}

.article-section h2 {
  font-size: 1.75rem;
  margin-bottom: 16px;
  padding-top: 24px;
  color: var(--text-color);
  border-top: 1px solid var(--border-color);
}

.article-section h3 {
  font-size: 1.25rem;
  margin: 24px 0 12px;
  color: var(--text-color);
}

.article-section p {
  margin-bottom: 16px;
  text-align: justify;
}

.article-section ul,
.article-section ol {
  margin: 16px 0;
  padding-left: 24px;
}

.article-section li {
  margin-bottom: 8px;
}

.article-section blockquote {
  margin: 24px 0;
  padding: 16px 24px;
  background: var(--bg-secondary);
  border-left: 4px solid var(--primary-color);
  font-style: italic;
  color: var(--text-light);
}

/* Navigation between articles */
.article-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 48px;
  padding-top: 24px;
  border-top: 1px solid var(--border-color);
}

.article-nav a {
  display: flex;
  flex-direction: column;
  padding: 16px;
  border: 1px solid var(--border-color);
  border-radius: 8px;
  max-width: 45%;
  transition: all 0.2s;
}

.article-nav a:hover {
  border-color: var(--primary-color);
  text-decoration: none;
}

.article-nav .nav-label {
  font-size: 0.75rem;
  text-transform: uppercase;
  color: var(--text-light);
  margin-bottom: 4px;
}

.article-nav .nav-title {
  font-weight: 500;
  color: var(--text-color);
}

/* ===== Footer ===== */
.footer {
  background: var(--bg-secondary);
  border-top: 1px solid var(--border-color);
  padding: 40px 24px;
  text-align: center;
  color: var(--text-light);
}

.footer p {
  margin-bottom: 8px;
}

.footer-title {
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--text-color);
  margin-bottom: 12px !important;
}

.footer-author {
  font-size: 1rem;
  color: var(--primary-color);
  padding: 12px 20px;
  background: rgba(26, 115, 232, 0.08);
  border-radius: 8px;
  display: inline-block;
  margin-bottom: 16px !important;
}

.footer-author strong {
  color: var(--primary-dark);
}

/* ===== Mobile Menu Button ===== */
.mobile-menu-btn {
  display: none;
  flex-direction: column;
  justify-content: center;
  gap: 5px;
  width: 40px;
  height: 40px;
  background: transparent;
  border: none;
  cursor: pointer;
  padding: 8px;
  margin-left: auto;
}

.mobile-menu-btn span {
  display: block;
  width: 24px;
  height: 2px;
  background: var(--text-color);
  border-radius: 2px;
  transition: all 0.3s;
}

/* ===== Responsive ===== */
@media (max-width: 1024px) {
  .sidebar {
    width: 240px;
  }
  
  .article-content {
    margin-left: 240px;
    padding: 32px 40px;
  }
}

@media (max-width: 768px) {
  /* Header Mobile */
  .header {
    padding: 0 16px;
  }
  
  .mobile-menu-btn {
    display: flex;
  }
  
  .logo-text {
    display: none;
  }
  
  .nav-main {
    position: fixed;
    top: var(--header-height);
    left: 0;
    right: 0;
    background: var(--bg-color);
    flex-direction: column;
    padding: 16px;
    border-bottom: 1px solid var(--border-color);
    box-shadow: 0 4px 12px rgba(0,0,0,0.1);
    display: none;
    gap: 0;
  }
  
  .nav-main.mobile-open {
    display: flex;
  }
  
  .nav-main a {
    padding: 16px;
    border-bottom: 1px solid var(--border-color);
    text-align: center;
    font-size: 1.1rem;
  }
  
  .nav-main a:last-child {
    border-bottom: none;
  }
  
  /* Hero Mobile */
  .hero {
    padding: 48px 16px;
  }
  
  .hero h1 {
    font-size: 1.75rem;
  }
  
  .hero p {
    font-size: 1rem;
  }
  
  /* Topics Grid Mobile */
  .topics-grid {
    grid-template-columns: 1fr;
    padding: 24px 0;
    gap: 16px;
  }
  
  .topic-card {
    padding: 20px;
  }
  
  .topic-card h2 {
    font-size: 1.1rem;
  }
  
  /* Sidebar Mobile */
  .article-layout {
    flex-direction: column;
  }
  
  .sidebar {
    position: static;
    width: 100%;
    border-right: none;
    border-bottom: 1px solid var(--border-color);
    max-height: none;
    padding: 16px 0;
  }
  
  .sidebar-header {
    padding: 0 16px 12px;
  }
  
  .toc-list {
    display: flex;
    flex-wrap: wrap;
    padding: 0 12px;
    gap: 4px;
  }
  
  .toc-list a {
    padding: 8px 12px;
    font-size: 0.8rem;
    background: var(--bg-color);
    border-radius: 20px;
    border: 1px solid var(--border-color);
  }
  
  .toc-list a:hover,
  .toc-list a.active {
    border-left: none;
    border-color: var(--primary-color);
  }
  
  .sidebar-footer {
    padding: 16px;
    margin-top: 8px;
  }
  
  .lang-toggle {
    flex-direction: row;
    flex-wrap: wrap;
    align-items: center;
  }
  
  .lang-label {
    width: 100%;
    margin-bottom: 4px;
  }
  
  .lang-btn {
    flex: 1;
    text-align: center;
    padding: 10px;
  }
  
  /* Article Content Mobile */
  .article-content {
    margin-left: 0;
    padding: 24px 16px;
    max-width: 100%;
  }
  
  .article-header {
    margin-bottom: 24px;
    padding-bottom: 16px;
  }
  
  .article-header h1 {
    font-size: 1.5rem;
    line-height: 1.3;
  }
  
  .article-meta {
    flex-direction: column;
    gap: 8px;
    font-size: 0.85rem;
  }
  
  .article-description {
    font-size: 1rem;
    padding: 16px;
    margin-bottom: 24px;
  }
  
  .article-section h2 {
    font-size: 1.35rem;
    padding-top: 20px;
  }
  
  .article-section p {
    font-size: 0.95rem;
    text-align: left;
  }
  
  .article-source {
    padding: 16px;
    margin-top: 32px;
  }
  
  .article-nav {
    flex-direction: column;
    gap: 12px;
    margin-top: 32px;
    padding-top: 20px;
  }
  
  .article-nav a {
    max-width: 100%;
    padding: 14px;
  }
  
  /* Footer Mobile */
  .footer {
    padding: 32px 16px;
  }
  
  .footer-title {
    font-size: 1.1rem;
  }
  
  .footer-author {
    font-size: 0.9rem;
    padding: 10px 16px;
    display: block;
  }
  
  /* Glossary Mobile */
  .glossary-item {
    flex-direction: column;
    gap: 8px;
    padding: 12px 0;
  }
  
  .glossary-en {
    flex: none;
    font-size: 1rem;
  }
  
  .glossary-vi {
    font-size: 0.9rem;
    color: var(--text-light);
  }
}

/* Extra small devices */
@media (max-width: 480px) {
  .hero h1 {
    font-size: 1.5rem;
  }
  
  .article-header h1 {
    font-size: 1.3rem;
  }
  
  .topic-card h2 {
    font-size: 1rem;
  }
  
  .toc-list a {
    font-size: 0.75rem;
    padding: 6px 10px;
  }
}

/* ===== Language Toggle ===== */
.sidebar-footer {
  padding: 20px;
  border-top: 1px solid var(--border-color);
  margin-top: 16px;
}

.lang-toggle {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.lang-label {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.5px;
  color: var(--text-light);
}

.lang-btn {
  padding: 8px 12px;
  border: 1px solid var(--border-color);
  border-radius: 6px;
  background: var(--bg-color);
  cursor: pointer;
  font-size: 0.85rem;
  transition: all 0.2s;
  text-align: left;
}

.lang-btn:hover {
  border-color: var(--primary-color);
  background: rgba(26, 115, 232, 0.05);
}

.lang-btn.active {
  background: var(--primary-color);
  color: white;
  border-color: var(--primary-color);
}

/* ===== Article Source ===== */
.article-source {
  margin-top: 40px;
  padding: 20px;
  background: var(--bg-secondary);
  border-radius: 8px;
  border-left: 4px solid var(--secondary-color);
}

.article-source p {
  margin: 0;
  font-size: 0.9rem;
}

/* ===== Glossary ===== */
.glossary-item {
  display: flex;
  padding: 16px 0;
  border-bottom: 1px solid var(--border-color);
  gap: 24px;
}

.glossary-item:last-child {
  border-bottom: none;
}

.glossary-en {
  flex: 0 0 280px;
  color: var(--primary-color);
}

.glossary-vi {
  flex: 1;
  color: var(--text-color);
}

.glossary-item.hidden {
  display: none;
}

/* ===== Article Description ===== */
.article-description {
  font-size: 1.1rem;
  color: var(--text-light);
  margin-bottom: 32px;
  padding: 20px;
  background: var(--bg-secondary);
  border-radius: 8px;
  line-height: 1.8;
}

.article-description p {
  margin: 0;
}

/* ===== Utilities ===== */
.text-center { text-align: center; }
.mt-4 { margin-top: 1rem; }
.mb-4 { margin-bottom: 1rem; }
.hidden { display: none; }

/* ===== Scroll Progress ===== */
.scroll-progress {
  position: fixed;
  top: var(--header-height);
  left: 0;
  height: 3px;
  background: var(--primary-color);
  z-index: 999;
  transition: width 0.1s;
}

/* ===== Animations ===== */
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

.topic-card {
  animation: fadeIn 0.4s ease-out;
}

.topic-card:nth-child(1) { animation-delay: 0.1s; }
.topic-card:nth-child(2) { animation-delay: 0.2s; }
.topic-card:nth-child(3) { animation-delay: 0.3s; }
.topic-card:nth-child(4) { animation-delay: 0.4s; }
.topic-card:nth-child(5) { animation-delay: 0.5s; }
.topic-card:nth-child(6) { animation-delay: 0.6s; }
.topic-card:nth-child(7) { animation-delay: 0.7s; }
.topic-card:nth-child(8) { animation-delay: 0.8s; }
.topic-card:nth-child(9) { animation-delay: 0.9s; }