/* ===========================
   CSS Variables & Theme
   =========================== */
:root {
  /* Colors - Dark theme by default */
  --color-bg-primary: #0a0a0f;
  --color-bg-secondary: #12121a;
  --color-bg-tertiary: #1a1a25;
  --color-bg-card: rgba(255, 255, 255, 0.03);
  --color-bg-card-hover: rgba(255, 255, 255, 0.06);
  
  --color-border: rgba(255, 255, 255, 0.08);
  --color-border-focus: rgba(99, 102, 241, 0.5);
  
  --color-text-primary: #ffffff;
  --color-text-secondary: rgba(255, 255, 255, 0.7);
  --color-text-tertiary: rgba(255, 255, 255, 0.4);
  
  --color-accent: #6366f1;
  --color-accent-hover: #818cf8;
  --color-accent-glow: rgba(99, 102, 241, 0.3);
  
  --color-success: #10b981;
  --color-success-bg: rgba(16, 185, 129, 0.1);
  
  --color-error: #ef4444;
  --color-error-bg: rgba(239, 68, 68, 0.1);
  
  --color-warning: #f59e0b;
  
  /* Gradients */
  --gradient-primary: linear-gradient(135deg, #6366f1 0%, #8b5cf6 50%, #a855f7 100%);
  --gradient-glow: radial-gradient(ellipse at center, rgba(99, 102, 241, 0.15) 0%, transparent 70%);
  
  /* Typography */
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 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;
  
  /* 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;
  
  /* Border radius */
  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1rem;
  --radius-2xl: 1.5rem;
  --radius-full: 9999px;
  
  /* Shadows */
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.3);
  --shadow-lg: 0 10px 15px rgba(0, 0, 0, 0.3);
  --shadow-xl: 0 20px 25px rgba(0, 0, 0, 0.4);
  --shadow-glow: 0 0 40px var(--color-accent-glow);
  
  /* Transitions */
  --transition-fast: 150ms ease;
  --transition-base: 200ms ease;
  --transition-slow: 300ms ease;
}

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

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

body {
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  line-height: 1.6;
  color: var(--color-text-primary);
  background: var(--color-bg-primary);
  min-height: 100vh;
  overflow-x: hidden;
}

/* ===========================
   App Layout
   =========================== */
.app-container {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
  position: relative;
}

.app-container::before {
  content: '';
  position: fixed;
  top: 0;
  left: 50%;
  transform: translateX(-50%);
  width: 100%;
  max-width: 800px;
  height: 400px;
  background: var(--gradient-glow);
  pointer-events: none;
  z-index: 0;
}

/* ===========================
   Header
   =========================== */
.header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: rgba(10, 10, 15, 0.8);
  backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--color-border);
}

.header-content {
  max-width: 800px;
  margin: 0 auto;
  padding: var(--space-4) var(--space-6);
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.logo {
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

.logo-icon {
  font-size: var(--font-size-2xl);
}

.logo-icon.large {
  font-size: 3rem;
}

.logo-text {
  font-size: var(--font-size-xl);
  font-weight: 700;
  background: var(--gradient-primary);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.user-info {
  display: flex;
  align-items: center;
  gap: var(--space-4);
}

.user-email {
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

/* ===========================
   Main Content
   =========================== */
.main-content {
  flex: 1;
  max-width: 800px;
  margin: 0 auto;
  padding: var(--space-8) var(--space-6);
  width: 100%;
  position: relative;
  z-index: 1;
}

/* ===========================
   Todo Container
   =========================== */
.todo-container {
  background: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-2xl);
  padding: var(--space-8);
  box-shadow: var(--shadow-xl);
  backdrop-filter: blur(10px);
}

/* ===========================
   Add Todo Form
   =========================== */
.add-todo-form {
  margin-bottom: var(--space-6);
}

.input-wrapper {
  display: flex;
  gap: var(--space-3);
}

.todo-input {
  flex: 1;
  padding: var(--space-4) var(--space-5);
  font-size: var(--font-size-base);
  font-family: var(--font-family);
  color: var(--color-text-primary);
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  outline: none;
  transition: all var(--transition-base);
}

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

.todo-input:focus {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px var(--color-accent-glow);
}

/* ===========================
   Buttons
   =========================== */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-5);
  font-size: var(--font-size-sm);
  font-weight: 500;
  font-family: var(--font-family);
  border-radius: var(--radius-lg);
  border: none;
  cursor: pointer;
  transition: all var(--transition-base);
  white-space: nowrap;
  text-decoration: none;
}

.btn:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.btn-primary {
  background: var(--gradient-primary);
  color: white;
  box-shadow: var(--shadow-md), var(--shadow-glow);
}

.btn-primary:hover:not(:disabled) {
  transform: translateY(-1px);
  box-shadow: var(--shadow-lg), 0 0 50px var(--color-accent-glow);
}

.btn-primary:active:not(:disabled) {
  transform: translateY(0);
}

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

.btn-ghost:hover:not(:disabled) {
  background: var(--color-bg-card-hover);
  color: var(--color-text-primary);
  border-color: var(--color-border-focus);
}

.btn-danger {
  background: var(--color-error-bg);
  color: var(--color-error);
}

.btn-danger:hover:not(:disabled) {
  background: var(--color-error);
  color: white;
}

.btn-icon {
  font-size: var(--font-size-lg);
  line-height: 1;
}

.btn-full {
  width: 100%;
}

.add-btn {
  padding: var(--space-4) var(--space-6);
}

/* ===========================
   Todo Stats
   =========================== */
.todo-stats {
  display: flex;
  gap: var(--space-4);
  margin-bottom: var(--space-6);
  padding-bottom: var(--space-4);
  border-bottom: 1px solid var(--color-border);
}

.stat {
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
}

.stat.secondary {
  color: var(--color-text-tertiary);
}

/* ===========================
   Todo List
   =========================== */
.todo-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.todo-item {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  transition: all var(--transition-base);
  animation: slideIn var(--transition-slow) ease-out;
}

@keyframes slideIn {
  from {
    opacity: 0;
    transform: translateY(-10px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.todo-item:hover {
  background: var(--color-bg-card-hover);
  border-color: var(--color-border-focus);
}

.todo-item.completed {
  opacity: 0.6;
}

.todo-item.completed .todo-title {
  text-decoration: line-through;
  color: var(--color-text-tertiary);
}

/* Checkbox */
.todo-checkbox {
  position: relative;
  width: 22px;
  height: 22px;
  flex-shrink: 0;
}

.todo-checkbox input {
  position: absolute;
  opacity: 0;
  width: 100%;
  height: 100%;
  cursor: pointer;
  z-index: 1;
}

.todo-checkbox .checkmark {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  border: 2px solid var(--color-border);
  border-radius: var(--radius-md);
  background: transparent;
  transition: all var(--transition-base);
}

.todo-checkbox input:checked + .checkmark {
  background: var(--gradient-primary);
  border-color: transparent;
}

.todo-checkbox .checkmark::after {
  content: '✓';
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%) scale(0);
  font-size: 12px;
  color: white;
  transition: all var(--transition-base);
}

.todo-checkbox input:checked + .checkmark::after {
  transform: translate(-50%, -50%) scale(1);
}

.todo-title {
  flex: 1;
  font-size: var(--font-size-base);
  color: var(--color-text-primary);
  word-break: break-word;
}

.todo-actions {
  display: flex;
  gap: var(--space-2);
  opacity: 0;
  transition: opacity var(--transition-base);
}

.todo-item:hover .todo-actions {
  opacity: 1;
}

.todo-delete-btn {
  padding: var(--space-2);
  font-size: var(--font-size-lg);
  background: transparent;
  border: none;
  color: var(--color-text-tertiary);
  cursor: pointer;
  border-radius: var(--radius-md);
  transition: all var(--transition-base);
}

.todo-delete-btn:hover {
  background: var(--color-error-bg);
  color: var(--color-error);
}

/* ===========================
   Empty & Loading States
   =========================== */
.empty-state,
.loading-state {
  text-align: center;
  padding: var(--space-12) var(--space-6);
  color: var(--color-text-tertiary);
}

.empty-icon {
  font-size: 4rem;
  margin-bottom: var(--space-4);
  opacity: 0.5;
}

.empty-state h3 {
  font-size: var(--font-size-xl);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-2);
}

.hidden {
  display: none !important;
}

/* Spinner */
.spinner {
  width: 40px;
  height: 40px;
  border: 3px solid var(--color-border);
  border-top-color: var(--color-accent);
  border-radius: 50%;
  margin: 0 auto var(--space-4);
  animation: spin 0.8s linear infinite;
}

@keyframes spin {
  to {
    transform: rotate(360deg);
  }
}

/* ===========================
   Footer
   =========================== */
.footer {
  text-align: center;
  padding: var(--space-6);
  color: var(--color-text-tertiary);
  font-size: var(--font-size-sm);
  border-top: 1px solid var(--color-border);
}

.cf-logo {
  color: var(--color-accent);
}

/* ===========================
   Auth Pages
   =========================== */
.auth-page {
  background: var(--color-bg-primary);
}

.auth-container {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: var(--space-6);
  position: relative;
  overflow: hidden;
}

.auth-card {
  width: 100%;
  max-width: 400px;
  background: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-2xl);
  padding: var(--space-10);
  backdrop-filter: blur(20px);
  box-shadow: var(--shadow-xl);
  position: relative;
  z-index: 1;
}

.auth-logo {
  text-align: center;
  margin-bottom: var(--space-8);
}

.auth-tagline {
  color: var(--color-text-tertiary);
  font-size: var(--font-size-sm);
  margin-top: var(--space-2);
}

.auth-form {
  display: flex;
  flex-direction: column;
  gap: var(--space-5);
}

.form-group {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.form-label {
  font-size: var(--font-size-sm);
  font-weight: 500;
  color: var(--color-text-secondary);
}

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

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

.form-input:focus {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px var(--color-accent-glow);
}

.form-hint {
  font-size: var(--font-size-xs);
  color: var(--color-text-tertiary);
}

.error-message {
  padding: var(--space-3) var(--space-4);
  background: var(--color-error-bg);
  border: 1px solid var(--color-error);
  border-radius: var(--radius-md);
  color: var(--color-error);
  font-size: var(--font-size-sm);
  text-align: center;
}

.auth-footer {
  text-align: center;
  margin-top: var(--space-6);
  padding-top: var(--space-6);
  border-top: 1px solid var(--color-border);
  color: var(--color-text-tertiary);
  font-size: var(--font-size-sm);
}

.auth-link {
  color: var(--color-accent);
  text-decoration: none;
  font-weight: 500;
  transition: color var(--transition-base);
}

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

/* Auth Background */
.auth-bg {
  position: fixed;
  inset: 0;
  pointer-events: none;
  overflow: hidden;
}

.auth-bg-gradient {
  position: absolute;
  top: -50%;
  left: -50%;
  width: 200%;
  height: 200%;
  background: 
    radial-gradient(ellipse at 20% 20%, rgba(99, 102, 241, 0.1) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 80%, rgba(139, 92, 246, 0.1) 0%, transparent 50%),
    radial-gradient(ellipse at 50% 50%, rgba(168, 85, 247, 0.05) 0%, transparent 50%);
  animation: gradientMove 20s ease-in-out infinite;
}

@keyframes gradientMove {
  0%, 100% {
    transform: translate(0, 0) rotate(0deg);
  }
  50% {
    transform: translate(-5%, -5%) rotate(180deg);
  }
}

.auth-bg-grid {
  position: absolute;
  inset: 0;
  background-image: 
    linear-gradient(rgba(255, 255, 255, 0.02) 1px, transparent 1px),
    linear-gradient(90deg, rgba(255, 255, 255, 0.02) 1px, transparent 1px);
  background-size: 50px 50px;
}

/* Button loader */
.btn-loader {
  display: inline-block;
  width: 16px;
  height: 16px;
  border: 2px solid rgba(255, 255, 255, 0.3);
  border-top-color: white;
  border-radius: 50%;
  animation: spin 0.6s linear infinite;
}

/* ===========================
   Responsive Design
   =========================== */
@media (max-width: 640px) {
  .header-content {
    padding: var(--space-3) var(--space-4);
  }
  
  .user-email {
    display: none;
  }
  
  .main-content {
    padding: var(--space-4);
  }
  
  .todo-container {
    padding: var(--space-5);
    border-radius: var(--radius-xl);
  }
  
  .input-wrapper {
    flex-direction: column;
  }
  
  .add-btn {
    width: 100%;
  }
  
  .todo-actions {
    opacity: 1;
  }
  
  .auth-card {
    padding: var(--space-6);
  }
}

/* ===========================
   Accessibility
   =========================== */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}

/* Focus visible for keyboard navigation */
:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}