/* ============================================
   DESIGN SYSTEM - Modern Minimal UI
   ============================================ */

/* CSS Custom Properties (Design Tokens) */
:root {
  /* Colors - Light Mode */
  --color-bg-primary: #ffffff;
  --color-bg-secondary: #f8fafc;
  --color-bg-tertiary: #f1f5f9;
  --color-bg-card: #ffffff;
  --color-bg-elevated: #ffffff;
  
  --color-text-primary: #0f172a;
  --color-text-secondary: #475569;
  --color-text-tertiary: #94a3b8;
  --color-text-inverse: #ffffff;
  
  --color-border: #e2e8f0;
  --color-border-light: #f1f5f9;
  --color-border-focus: #3b82f6;
  
  --color-accent: #3b82f6;
  --color-accent-hover: #2563eb;
  --color-accent-light: #eff6ff;
  --color-accent-subtle: #dbeafe;
  
  --color-success: #10b981;
  --color-success-light: #d1fae5;
  --color-warning: #f59e0b;
  --color-warning-light: #fef3c7;
  --color-error: #ef4444;
  --color-error-light: #fee2e2;
  
  /* Subject Colors */
  --color-english: #3b82f6;
  --color-english-light: #dbeafe;
  --color-german: #f59e0b;
  --color-german-light: #fef3c7;
  
  /* Shadows */
  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.04);
  --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.04), 0 1px 2px rgba(0, 0, 0, 0.06);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.05), 0 2px 4px rgba(0, 0, 0, 0.04);
  --shadow-lg: 0 12px 40px rgba(0, 0, 0, 0.08), 0 4px 12px rgba(0, 0, 0, 0.04);
  --shadow-xl: 0 24px 60px rgba(0, 0, 0, 0.12), 0 8px 24px rgba(0, 0, 0, 0.06);
  --shadow-card: 0 1px 3px rgba(0, 0, 0, 0.04), 0 4px 20px rgba(0, 0, 0, 0.04);
  --shadow-card-hover: 0 8px 40px rgba(0, 0, 0, 0.08), 0 4px 12px rgba(0, 0, 0, 0.04);
  
  /* Typography */
  --font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-size-xs: 0.75rem;
  --font-size-sm: 0.875rem;
  --font-size-base: 1rem;
  --font-size-lg: 1.125rem;
  --font-size-xl: 1.25rem;
  --font-size-2xl: 1.5rem;
  --font-size-3xl: 2rem;
  --font-size-4xl: 2.5rem;
  
  --font-weight-normal: 400;
  --font-weight-medium: 500;
  --font-weight-semibold: 600;
  --font-weight-bold: 700;
  
  --line-height-tight: 1.2;
  --line-height-normal: 1.5;
  --line-height-relaxed: 1.75;
  
  --letter-spacing-tight: -0.025em;
  --letter-spacing-normal: 0;
  --letter-spacing-wide: 0.025em;
  
  /* Spacing */
  --space-1: 0.25rem;
  --space-2: 0.5rem;
  --space-3: 0.75rem;
  --space-4: 1rem;
  --space-5: 1.25rem;
  --space-6: 1.5rem;
  --space-8: 2rem;
  --space-10: 2.5rem;
  --space-12: 3rem;
  --space-16: 4rem;
  --space-20: 5rem;
  
  /* Border Radius */
  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;
  --radius-2xl: 1.5rem;
  --radius-full: 9999px;
  
  /* Transitions */
  --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-base: 200ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-slow: 300ms cubic-bezier(0.4, 0, 0.2, 1);
  --transition-spring: 500ms cubic-bezier(0.34, 1.56, 0.64, 1);
  
  /* Z-Index */
  --z-dropdown: 100;
  --z-sticky: 200;
  --z-modal: 300;
  --z-tooltip: 400;
}

/* Dark Mode */
[data-theme="dark"] {
  --color-bg-primary: #0f172a;
  --color-bg-secondary: #1e293b;
  --color-bg-tertiary: #334155;
  --color-bg-card: #1e293b;
  --color-bg-elevated: #334155;
  
  --color-text-primary: #f8fafc;
  --color-text-secondary: #cbd5e1;
  --color-text-tertiary: #64748b;
  --color-text-inverse: #0f172a;
  
  --color-border: #334155;
  --color-border-light: #1e293b;
  --color-border-focus: #60a5fa;
  
  --color-accent: #60a5fa;
  --color-accent-hover: #3b82f6;
  --color-accent-light: #1e3a5f;
  --color-accent-subtle: #1e3a5f;
  
  --color-success-light: #064e3b;
  --color-warning-light: #78350f;
  --color-error-light: #7f1d1d;
  
  --color-english-light: #1e3a5f;
  --color-german-light: #78350f;
  
  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.2);
  --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.2), 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.25), 0 2px 4px rgba(0, 0, 0, 0.2);
  --shadow-lg: 0 12px 40px rgba(0, 0, 0, 0.3), 0 4px 12px rgba(0, 0, 0, 0.2);
  --shadow-xl: 0 24px 60px rgba(0, 0, 0, 0.4), 0 8px 24px rgba(0, 0, 0, 0.25);
  --shadow-card: 0 1px 3px rgba(0, 0, 0, 0.2), 0 4px 20px rgba(0, 0, 0, 0.15);
  --shadow-card-hover: 0 8px 40px rgba(0, 0, 0, 0.3), 0 4px 12px rgba(0, 0, 0, 0.2);
}

/* ============================================
   BASE STYLES
   ============================================ */

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

html {
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  margin: 0;
  padding: 0;
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  line-height: var(--line-height-normal);
  color: var(--color-text-primary);
  background-color: var(--color-bg-secondary);
  transition: background-color var(--transition-slow), color var(--transition-slow);
}

/* ============================================
   TYPOGRAPHY
   ============================================ */

h1, h2, h3, h4, h5, h6 {
  margin: 0 0 var(--space-4);
  font-weight: var(--font-weight-semibold);
  line-height: var(--line-height-tight);
  letter-spacing: var(--letter-spacing-tight);
  color: var(--color-text-primary);
}

h1 { font-size: var(--font-size-4xl); }
h2 { font-size: var(--font-size-3xl); }
h3 { font-size: var(--font-size-2xl); }
h4 { font-size: var(--font-size-xl); }

p {
  margin: 0 0 var(--space-4);
  color: var(--color-text-secondary);
}

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

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

/* ============================================
   LAYOUT UTILITIES
   ============================================ */

.page-wrapper {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.page-center {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-6);
}

.container {
  width: 100%;
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 var(--space-6);
}

.container-sm {
  max-width: 480px;
}

.container-md {
  max-width: 640px;
}

.container-lg {
  max-width: 800px;
}

/* ============================================
   CARD COMPONENT
   ============================================ */

.card {
  background: var(--color-bg-card);
  border-radius: var(--radius-2xl);
  box-shadow: var(--shadow-card);
  padding: var(--space-8);
  transition: transform var(--transition-base), box-shadow var(--transition-base);
}

.card-hover:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-card-hover);
}

.card-compact {
  padding: var(--space-6);
}

.card-header {
  margin-bottom: var(--space-6);
}

.card-title {
  margin-bottom: var(--space-2);
}

.card-subtitle {
  color: var(--color-text-secondary);
  font-size: var(--font-size-base);
}

/* ============================================
   BUTTON COMPONENT
   ============================================ */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-6);
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  font-weight: var(--font-weight-medium);
  line-height: 1;
  text-decoration: none;
  border: none;
  border-radius: var(--radius-lg);
  cursor: pointer;
  transition: all var(--transition-fast);
  outline: none;
  position: relative;
  overflow: hidden;
}

.btn:focus-visible {
  box-shadow: 0 0 0 3px var(--color-accent-subtle);
}

.btn:active {
  transform: scale(0.98);
}

/* Button Variants */
.btn-primary {
  background: var(--color-accent);
  color: var(--color-text-inverse);
}

.btn-primary:hover {
  background: var(--color-accent-hover);
  color: var(--color-text-inverse);
}

.btn-secondary {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
}

.btn-secondary:hover {
  background: var(--color-border);
}

.btn-ghost {
  background: transparent;
  color: var(--color-text-secondary);
}

.btn-ghost:hover {
  background: var(--color-bg-tertiary);
  color: var(--color-text-primary);
}

.btn-outline {
  background: transparent;
  border: 1px solid var(--color-border);
  color: var(--color-text-primary);
}

.btn-outline:hover {
  background: var(--color-bg-tertiary);
  border-color: var(--color-border);
}

/* Button Sizes */
.btn-sm {
  padding: var(--space-2) var(--space-4);
  font-size: var(--font-size-sm);
}

.btn-lg {
  padding: var(--space-4) var(--space-8);
  font-size: var(--font-size-lg);
}

.btn-block {
  width: 100%;
}

/* Button with Icon */
.btn-icon {
  padding: var(--space-3);
  aspect-ratio: 1;
}

.btn-icon-sm {
  padding: var(--space-2);
}

/* ============================================
   INPUT COMPONENT
   ============================================ */

.input-group {
  margin-bottom: var(--space-4);
}

.input-label {
  display: block;
  margin-bottom: var(--space-2);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
}

.input-wrapper {
  position: relative;
  display: flex;
  align-items: center;
}

.input-icon {
  position: absolute;
  left: var(--space-4);
  width: 20px;
  height: 20px;
  color: var(--color-text-tertiary);
  pointer-events: none;
  transition: color var(--transition-fast);
}

.input {
  width: 100%;
  padding: var(--space-4);
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  color: var(--color-text-primary);
  background: var(--color-bg-tertiary);
  border: 1px solid transparent;
  border-radius: var(--radius-lg);
  outline: none;
  transition: all var(--transition-fast);
}

.input::placeholder {
  color: var(--color-text-tertiary);
}

.input:hover {
  background: var(--color-bg-secondary);
  border-color: var(--color-border);
}

.input:focus {
  background: var(--color-bg-primary);
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px var(--color-accent-subtle);
}

.input-with-icon {
  padding-left: calc(var(--space-4) + 20px + var(--space-3));
}

.input-wrapper:focus-within .input-icon {
  color: var(--color-accent);
}

/* Error State */
.input-error {
  border-color: var(--color-error);
}

.input-error:focus {
  border-color: var(--color-error);
  box-shadow: 0 0 0 3px var(--color-error-light);
}

.error-message {
  margin-top: var(--space-2);
  font-size: var(--font-size-sm);
  color: var(--color-error);
}

/* ============================================
   NAVIGATION COMPONENT
   ============================================ */

.nav-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: var(--z-sticky);
  background: var(--color-bg-card);
  border-bottom: 1px solid var(--color-border);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
}

.nav-container {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 64px;
  padding: 0 var(--space-6);
  max-width: 1400px;
  margin: 0 auto;
}

.nav-brand {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-bold);
  color: var(--color-text-primary);
  text-decoration: none;
}

.nav-links {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  list-style: none;
  margin: 0;
  padding: 0;
}

.nav-link {
  padding: var(--space-2) var(--space-4);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
  text-decoration: none;
  border-radius: var(--radius-md);
  transition: all var(--transition-fast);
}

.nav-link:hover {
  color: var(--color-text-primary);
  background: var(--color-bg-tertiary);
}

.nav-link.active {
  color: var(--color-accent);
  background: var(--color-accent-light);
}

.nav-actions {
  display: flex;
  align-items: center;
  gap: var(--space-3);
}

/* Mobile Navigation */
.nav-mobile-toggle {
  display: none;
  padding: var(--space-2);
  background: transparent;
  border: none;
  cursor: pointer;
  color: var(--color-text-primary);
}

.nav-mobile-toggle svg {
  width: 24px;
  height: 24px;
}

@media (max-width: 768px) {
  .nav-links {
    position: fixed;
    top: 64px;
    left: 0;
    right: 0;
    bottom: 0;
    flex-direction: column;
    padding: var(--space-6);
    background: var(--color-bg-card);
    border-top: 1px solid var(--color-border);
    transform: translateX(-100%);
    opacity: 0;
    transition: transform var(--transition-slow), opacity var(--transition-slow);
    pointer-events: none;
  }
  
  .nav-links.open {
    transform: translateX(0);
    opacity: 1;
    pointer-events: all;
  }
  
  .nav-link {
    width: 100%;
    padding: var(--space-4);
    font-size: var(--font-size-lg);
  }
  
  .nav-mobile-toggle {
    display: flex;
  }
  
  .nav-actions {
    display: none;
  }
  
  .nav-links.open + .nav-actions {
    display: flex;
    position: fixed;
    bottom: var(--space-8);
    left: var(--space-6);
    right: var(--space-6);
  }
}

/* ============================================
   THEME TOGGLE
   ============================================ */

.theme-toggle {
  position: relative;
  width: 44px;
  height: 44px;
  padding: 0;
  background: var(--color-bg-tertiary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  cursor: pointer;
  transition: all var(--transition-fast);
  display: flex;
  align-items: center;
  justify-content: center;
}

.theme-toggle:hover {
  background: var(--color-bg-secondary);
  border-color: var(--color-border);
}

.theme-toggle svg {
  width: 20px;
  height: 20px;
  color: var(--color-text-secondary);
  transition: transform var(--transition-base), opacity var(--transition-base);
}

.theme-toggle .icon-sun {
  position: absolute;
  opacity: 0;
  transform: rotate(-90deg) scale(0.5);
}

.theme-toggle .icon-moon {
  opacity: 1;
  transform: rotate(0) scale(1);
}

[data-theme="dark"] .theme-toggle .icon-sun {
  opacity: 1;
  transform: rotate(0) scale(1);
}

[data-theme="dark"] .theme-toggle .icon-moon {
  opacity: 0;
  transform: rotate(90deg) scale(0.5);
}

/* ============================================
   SUBJECT CARDS
   ============================================ */

.subject-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: var(--space-6);
  margin-top: var(--space-8);
}

.subject-card {
  position: relative;
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: var(--space-10) var(--space-6);
  background: var(--color-bg-card);
  border-radius: var(--radius-2xl);
  box-shadow: var(--shadow-card);
  text-decoration: none;
  overflow: hidden;
  transition: all var(--transition-base);
}

.subject-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 4px;
  background: var(--subject-color, var(--color-accent));
  transform: scaleX(0);
  transition: transform var(--transition-base);
}

.subject-card:hover {
  transform: translateY(-8px);
  box-shadow: var(--shadow-card-hover);
}

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

.subject-icon {
  width: 80px;
  height: 80px;
  margin-bottom: var(--space-6);
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-xl);
  background: var(--subject-bg, var(--color-accent-light));
  transition: transform var(--transition-spring);
}

.subject-card:hover .subject-icon {
  transform: scale(1.1);
}

.subject-icon svg {
  width: 40px;
  height: 40px;
  color: var(--subject-color, var(--color-accent));
}

.subject-title {
  margin: 0;
  font-size: var(--font-size-xl);
  font-weight: var(--font-weight-semibold);
  color: var(--color-text-primary);
}

.subject-card.english {
  --subject-color: var(--color-english);
  --subject-bg: var(--color-english-light);
}

.subject-card.german {
  --subject-color: var(--color-german);
  --subject-bg: var(--color-german-light);
}

/* ============================================
   MATERIAL LINK LIST
   ============================================ */

.material-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.material-link {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  background: var(--color-bg-tertiary);
  border-radius: var(--radius-lg);
  text-decoration: none;
  color: var(--color-text-primary);
  transition: all var(--transition-fast);
}

.material-link:hover {
  background: var(--color-accent-light);
  color: var(--color-accent);
  transform: translateX(4px);
}

.material-link-icon {
  width: 44px;
  height: 44px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--color-bg-card);
  border-radius: var(--radius-md);
  flex-shrink: 0;
}

.material-link-icon svg {
  width: 20px;
  height: 20px;
  color: var(--color-text-tertiary);
  transition: color var(--transition-fast);
}

.material-link:hover .material-link-icon svg {
  color: var(--color-accent);
}

.material-link-text {
  font-weight: var(--font-weight-medium);
  flex-grow: 1;
}

.material-link-arrow {
  width: 20px;
  height: 20px;
  color: var(--color-text-tertiary);
  transition: transform var(--transition-fast), color var(--transition-fast);
}

.material-link:hover .material-link-arrow {
  transform: translateX(4px);
  color: var(--color-accent);
}

/* ============================================
   BREADCRUMB
   ============================================ */

.breadcrumb {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin-bottom: var(--space-6);
  font-size: var(--font-size-sm);
}

.breadcrumb-link {
  color: var(--color-text-tertiary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

.breadcrumb-link:hover {
  color: var(--color-accent);
}

.breadcrumb-separator {
  color: var(--color-text-tertiary);
}

.breadcrumb-current {
  color: var(--color-text-primary);
  font-weight: var(--font-weight-medium);
}

/* ============================================
   WELCOME SECTION
   ============================================ */

.welcome-section {
  text-align: center;
  padding: var(--space-12) 0 var(--space-8);
}

.welcome-greeting {
  font-size: var(--font-size-lg);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-2);
}

.welcome-name {
  font-size: var(--font-size-4xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-text-primary);
  margin: 0;
}

.welcome-subtitle {
  margin-top: var(--space-4);
  font-size: var(--font-size-lg);
  color: var(--color-text-secondary);
}

/* ============================================
   BACK LINK
   ============================================ */

.back-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  margin-bottom: var(--space-6);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

.back-link:hover {
  color: var(--color-accent);
}

.back-link svg {
  width: 16px;
  height: 16px;
  transition: transform var(--transition-fast);
}

.back-link:hover svg {
  transform: translateX(-4px);
}

/* ============================================
   PAGE TRANSITIONS
   ============================================ */

.fade-in {
  animation: fadeIn var(--transition-slow) ease-out;
}

.slide-up {
  animation: slideUp var(--transition-slow) ease-out;
}

.stagger > * {
  animation: slideUp var(--transition-slow) ease-out backwards;
}

.stagger > *:nth-child(1) { animation-delay: 0ms; }
.stagger > *:nth-child(2) { animation-delay: 50ms; }
.stagger > *:nth-child(3) { animation-delay: 100ms; }
.stagger > *:nth-child(4) { animation-delay: 150ms; }
.stagger > *:nth-child(5) { animation-delay: 200ms; }
.stagger > *:nth-child(6) { animation-delay: 250ms; }

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

@keyframes slideUp {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

@keyframes scaleIn {
  from {
    opacity: 0;
    transform: scale(0.95);
  }
  to {
    opacity: 1;
    transform: scale(1);
  }
}

/* ============================================
   RESPONSIVE UTILITIES
   ============================================ */

@media (max-width: 640px) {
  html {
    font-size: 14px;
  }
  
  .card {
    padding: var(--space-6);
    border-radius: var(--radius-xl);
  }
  
  .welcome-name {
    font-size: var(--font-size-3xl);
  }
  
  .subject-grid {
    grid-template-columns: 1fr;
    gap: var(--space-4);
  }
  
  .subject-card {
    padding: var(--space-8) var(--space-4);
  }
  
  .subject-icon {
    width: 64px;
    height: 64px;
  }
  
  .subject-icon svg {
    width: 32px;
    height: 32px;
  }
}

/* ============================================
   FOCUS STYLES (Accessibility)
   ============================================ */

:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}

/* Remove outline for mouse users */
:focus:not(:focus-visible) {
  outline: none;
}

/* ============================================
   SCROLLBAR STYLING
   ============================================ */

::-webkit-scrollbar {
  width: 8px;
  height: 8px;
}

::-webkit-scrollbar-track {
  background: var(--color-bg-secondary);
}

::-webkit-scrollbar-thumb {
  background: var(--color-border);
  border-radius: var(--radius-full);
}

::-webkit-scrollbar-thumb:hover {
  background: var(--color-text-tertiary);
}

/* ============================================
   SELECTION STYLING
   ============================================ */

::selection {
  background: var(--color-accent-subtle);
  color: var(--color-text-primary);
}