/* Design Tokens & CSS Reset */
:root {
  /* Strict 4pt / 8pt Spacing Scale */
  --space-4: 4px;
  --space-8: 8px;
  --space-12: 12px;
  --space-16: 16px;
  --space-20: 20px;
  --space-24: 24px;
  --space-32: 32px;
  --space-40: 40px;
  --space-48: 48px;
  --space-64: 64px;

  /* Purposeful Coral Palette */
  --color-coral: #E15B45;
  --color-coral-hover: #CC4D38;
  --color-coral-light: #FEF2F0;
  --color-coral-border: #FCD1CA;
  --color-coral-ring: rgba(225, 91, 69, 0.2);

  /* Neutrals & Surfaces */
  --color-bg: #F8F9FA;
  --color-surface: #FFFFFF;
  --color-border: #E4E4E7;
  --color-border-subtle: #F4F4F5;
  --color-text: #18181B;
  --color-text-muted: #71717A;
  --color-text-subtle: #A1A1AA;

  /* Feedback Colors */
  --color-danger: #DC2626;
  --color-danger-hover: #B91C1C;
  --color-danger-light: #FEF2F2;
  --color-danger-border: #FECACA;
  --color-success: #16A34A;
  --color-success-light: #F0FDF4;

  /* Consistent Component Radii & Shadows */
  --radius-sm: 6px;
  --radius-md: 8px;
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-card: 0 4px 6px -1px rgba(0, 0, 0, 0.05), 0 2px 4px -2px rgba(0, 0, 0, 0.05);

  /* Typography Scale */
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  --font-xs: 12px;
  --line-xs: 16px;
  --font-sm: 14px;
  --line-sm: 20px;
  --font-base: 16px;
  --line-base: 24px;
  --font-md: 20px;
  --line-md: 28px;
  --font-lg: 24px;
  --line-lg: 32px;
}

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: var(--font-family);
  background-color: var(--color-bg);
  color: var(--color-text);
  font-size: var(--font-base);
  line-height: var(--line-base);
  -webkit-font-smoothing: antialiased;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* Screen-reader utility */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border-width: 0;
}

.hidden {
  display: none !important;
}

/* Main Layout */
.app-layout {
  width: 100%;
  max-width: 640px;
  margin: 0 auto;
  padding: var(--space-48) var(--space-16) var(--space-32);
  display: flex;
  flex-direction: column;
  flex: 1;
  gap: var(--space-24);
}

/* Header */
.app-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.brand {
  display: flex;
  align-items: center;
  gap: var(--space-12);
}

.brand-icon {
  width: 36px;
  height: 36px;
  background-color: var(--color-coral);
  color: #FFFFFF;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: var(--shadow-sm);
}

.brand-title {
  font-size: var(--font-lg);
  line-height: var(--line-lg);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--color-text);
}

.brand-subtitle {
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
}

.status-indicator {
  display: inline-flex;
  align-items: center;
  gap: var(--space-8);
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
  background-color: var(--color-surface);
  border: 1px solid var(--color-border);
  padding: var(--space-4) var(--space-12);
  border-radius: var(--radius-sm);
}

.status-dot {
  width: 8px;
  height: 8px;
  border-radius: 50%;
  background-color: var(--color-success);
  transition: background-color 0.2s ease;
}

.status-indicator.is-syncing .status-dot {
  background-color: var(--color-coral);
  animation: pulse 1s infinite;
}

.status-indicator.is-error .status-dot {
  background-color: var(--color-danger);
}

@keyframes pulse {
  0% { opacity: 1; }
  50% { opacity: 0.3; }
  100% { opacity: 1; }
}

/* Card */
.card {
  background-color: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-card);
  overflow: hidden;
  display: flex;
  flex-direction: column;
}

/* Banner */
.banner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-12) var(--space-16);
  background-color: var(--color-danger-light);
  border-bottom: 1px solid var(--color-danger-border);
  color: var(--color-danger);
  font-size: var(--font-sm);
  line-height: var(--line-sm);
}

.banner.success {
  background-color: var(--color-success-light);
  border-bottom-color: #BBF7D0;
  color: var(--color-success);
}

.banner-close {
  background: none;
  border: none;
  color: inherit;
  font-size: 18px;
  line-height: 1;
  cursor: pointer;
  padding: 0 var(--space-4);
}

/* Add Task Form */
.todo-form {
  display: flex;
  gap: var(--space-8);
  padding: var(--space-16);
  border-bottom: 1px solid var(--color-border-subtle);
}

.input-field {
  flex: 1;
  height: 40px;
  padding: var(--space-8) var(--space-12);
  font-family: inherit;
  font-size: var(--font-base);
  line-height: var(--line-base);
  color: var(--color-text);
  background-color: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  outline: none;
  transition: border-color 0.15s ease, box-shadow 0.15s ease;
}

.input-field:focus {
  border-color: var(--color-coral);
  box-shadow: 0 0 0 3px var(--color-coral-ring);
}

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

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  height: 40px;
  padding: 0 var(--space-16);
  font-family: inherit;
  font-size: var(--font-sm);
  line-height: var(--line-sm);
  font-weight: 500;
  border-radius: var(--radius-md);
  border: 1px solid transparent;
  cursor: pointer;
  transition: all 0.15s ease;
  user-select: none;
  white-space: nowrap;
}

.btn-primary {
  background-color: var(--color-coral);
  color: #FFFFFF;
  box-shadow: var(--shadow-sm);
}

.btn-primary:hover:not(:disabled) {
  background-color: var(--color-coral-hover);
}

.btn-primary:active:not(:disabled) {
  transform: translateY(1px);
}

.btn-primary:disabled {
  opacity: 0.6;
  cursor: not-allowed;
}

.btn-ghost {
  height: 32px;
  padding: 0 var(--space-8);
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
  background: transparent;
}

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

.btn-ghost:disabled {
  opacity: 0.35;
  cursor: not-allowed;
}

.btn-spinner {
  width: 14px;
  height: 14px;
  border: 2px solid rgba(255, 255, 255, 0.4);
  border-radius: 50%;
  border-top-color: #FFFFFF;
  animation: spin 0.6s linear infinite;
  display: inline-block;
}

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

/* Filter Controls Bar */
.controls-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-8) var(--space-16);
  background-color: var(--color-bg);
  border-bottom: 1px solid var(--color-border-subtle);
}

.filter-group {
  display: flex;
  gap: var(--space-4);
}

.filter-tab {
  display: inline-flex;
  align-items: center;
  gap: var(--space-8);
  height: 32px;
  padding: 0 var(--space-12);
  font-family: inherit;
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  font-weight: 500;
  color: var(--color-text-muted);
  background: transparent;
  border: 1px solid transparent;
  border-radius: var(--radius-sm);
  cursor: pointer;
  transition: all 0.15s ease;
}

.filter-tab:hover {
  color: var(--color-text);
  background-color: rgba(0, 0, 0, 0.04);
}

.filter-tab.active {
  color: var(--color-coral);
  background-color: var(--color-surface);
  border-color: var(--color-coral-border);
  box-shadow: var(--shadow-sm);
}

.counter {
  font-size: var(--font-xs);
  padding: 1px 6px;
  border-radius: var(--radius-sm);
  background-color: var(--color-border-subtle);
  color: var(--color-text-muted);
}

.filter-tab.active .counter {
  background-color: var(--color-coral-light);
  color: var(--color-coral);
}

/* Skeleton Loading State */
.skeleton-list {
  padding: var(--space-16);
  display: flex;
  flex-direction: column;
  gap: var(--space-12);
}

.skeleton-item {
  display: flex;
  align-items: center;
  gap: var(--space-12);
  padding: var(--space-8) 0;
}

.skeleton-box {
  width: 20px;
  height: 20px;
  border-radius: var(--radius-sm);
  background: linear-gradient(90deg, #F4F4F5 25%, #E4E4E7 50%, #F4F4F5 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
}

.skeleton-line {
  height: 16px;
  flex: 1;
  border-radius: var(--radius-sm);
  background: linear-gradient(90deg, #F4F4F5 25%, #E4E4E7 50%, #F4F4F5 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
}

.skeleton-line.short { width: 45%; flex: none; }
.skeleton-line.medium { width: 70%; flex: none; }

@keyframes shimmer {
  0% { background-position: 200% 0; }
  100% { background-position: -200% 0; }
}

/* Todo List */
.todo-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  max-height: 480px;
  overflow-y: auto;
}

.todo-item {
  display: flex;
  align-items: center;
  gap: var(--space-12);
  padding: var(--space-12) var(--space-16);
  border-bottom: 1px solid var(--color-border-subtle);
  background-color: var(--color-surface);
  transition: background-color 0.15s ease, opacity 0.15s ease;
}

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

.todo-item:hover {
  background-color: #FAFAFA;
}

.todo-item.is-busy {
  opacity: 0.5;
  pointer-events: none;
}

/* Checkbox */
.custom-checkbox {
  position: relative;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 20px;
  height: 20px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  background-color: var(--color-surface);
  cursor: pointer;
  flex-shrink: 0;
  transition: all 0.15s ease;
}

.custom-checkbox:hover {
  border-color: var(--color-coral);
}

.todo-item.is-completed .custom-checkbox {
  background-color: var(--color-coral);
  border-color: var(--color-coral);
}

.checkbox-icon {
  display: none;
  color: #FFFFFF;
}

.todo-item.is-completed .checkbox-icon {
  display: block;
}

/* Todo Title & Inline Edit */
.todo-content {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
  min-width: 0;
}

.todo-title {
  font-size: var(--font-base);
  line-height: var(--line-base);
  color: var(--color-text);
  word-break: break-word;
  cursor: pointer;
  transition: color 0.15s ease;
}

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

.todo-meta {
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
}

.edit-input {
  width: 100%;
  height: 32px;
  padding: var(--space-4) var(--space-8);
  font-family: inherit;
  font-size: var(--font-base);
  line-height: var(--line-base);
  color: var(--color-text);
  background-color: var(--color-surface);
  border: 1px solid var(--color-coral);
  border-radius: var(--radius-sm);
  box-shadow: 0 0 0 3px var(--color-coral-ring);
  outline: none;
}

/* Action Buttons */
.item-actions {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  opacity: 0.4;
  transition: opacity 0.15s ease;
}

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

.icon-btn {
  width: 28px;
  height: 28px;
  padding: 0;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border: 1px solid transparent;
  border-radius: var(--radius-sm);
  background: transparent;
  color: var(--color-text-muted);
  cursor: pointer;
  transition: all 0.15s ease;
}

.icon-btn:hover {
  background-color: var(--color-border-subtle);
  color: var(--color-text);
}

.icon-btn-delete:hover {
  background-color: var(--color-danger-light);
  color: var(--color-danger);
}

/* Empty State */
.empty-state {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: var(--space-48) var(--space-24);
  gap: var(--space-12);
  text-align: center;
}

.empty-icon {
  color: var(--color-text-subtle);
}

.empty-text {
  font-size: var(--font-sm);
  line-height: var(--line-sm);
  color: var(--color-text-muted);
}

/* Card Footer */
.card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-12) var(--space-16);
  background-color: var(--color-bg);
  border-top: 1px solid var(--color-border-subtle);
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
}

/* App Footer */
.app-footer {
  text-align: center;
  font-size: var(--font-xs);
  line-height: var(--line-xs);
  color: var(--color-text-muted);
  margin-top: auto;
}

/* Responsive adjustments */
@media (max-width: 480px) {
  .app-layout {
    padding: var(--space-24) var(--space-12);
  }

  .todo-form {
    flex-direction: column;
  }

  .btn-primary {
    width: 100%;
  }

  .item-actions {
    opacity: 1;
  }
}