/* =========================================================================
   Design system — CSS custom properties
   Light theme by default; dark variants are redefined under body.dark-mode.
   ========================================================================= */
:root {
  /* Brand / accent */
  --accent: #6366f1;
  --accent-2: #8b5cf6;
  --accent-strong: #4f46e5;
  --accent-soft: rgba(99, 102, 241, 0.12);

  /* Semantic */
  --success: #10b981;
  --success-strong: #059669;
  --danger: #ef4444;
  --danger-strong: #dc2626;

  /* Surfaces & text */
  --bg-1: #eef2ff;
  --bg-2: #faf5ff;
  --bg-3: #ecfeff;
  --surface: #ffffff;
  --surface-2: #f1f5f9;
  --surface-hover: #f8fafc;
  --text: #0f172a;
  --muted: #64748b;
  --border: #e7e9f3;
  --border-strong: #d6dae8;

  /* Effects */
  --radius: 16px;
  --radius-sm: 10px;
  --radius-pill: 999px;
  --shadow-sm: 0 1px 2px rgba(15, 23, 42, 0.06);
  --shadow-md: 0 8px 24px -12px rgba(15, 23, 42, 0.25);
  --shadow-lg: 0 24px 60px -28px rgba(79, 70, 229, 0.45);
  --ring: 0 0 0 3px var(--accent-soft);
  --ease: cubic-bezier(0.22, 1, 0.36, 1);
}

body.dark-mode {
  --accent: #818cf8;
  --accent-2: #a78bfa;
  --accent-strong: #6366f1;
  --accent-soft: rgba(129, 140, 248, 0.18);

  --success: #34d399;
  --success-strong: #10b981;
  --danger: #f87171;
  --danger-strong: #ef4444;

  --bg-1: #0b1120;
  --bg-2: #14112e;
  --bg-3: #0a1626;
  --surface: #0f172a;
  --surface-2: #1e293b;
  --surface-hover: #1c2740;
  --text: #e2e8f0;
  --muted: #94a3b8;
  --border: #243049;
  --border-strong: #334155;

  --shadow-md: 0 10px 30px -14px rgba(0, 0, 0, 0.65);
  --shadow-lg: 0 28px 70px -30px rgba(0, 0, 0, 0.8);
}

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

body {
  margin: 0;
  min-height: 100vh;
  padding: clamp(1.25rem, 4vw, 3rem) 1.25rem;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  font-family: "Inter", system-ui, -apple-system, "Segoe UI", sans-serif;
  color: var(--text);
  line-height: 1.5;
  background:
    radial-gradient(1200px 600px at 12% -10%, var(--bg-2), transparent 60%),
    radial-gradient(1000px 500px at 110% 10%, var(--bg-3), transparent 55%),
    var(--bg-1);
  background-attachment: fixed;
  transition: background-color 0.4s var(--ease), color 0.4s var(--ease);
  -webkit-font-smoothing: antialiased;
}

/* =========================================================================
   App card
   ========================================================================= */
.app {
  width: 100%;
  max-width: 600px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: calc(var(--radius) + 8px);
  box-shadow: var(--shadow-lg);
  padding: clamp(1.5rem, 4vw, 2.5rem);
  animation: rise 0.5s var(--ease) both;
}

@keyframes rise {
  from { opacity: 0; transform: translateY(12px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Header */
.app-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  margin-bottom: 1.75rem;
}

#app-title {
  margin: 0;
  font-size: clamp(1.6rem, 5vw, 2.1rem);
  font-weight: 800;
  letter-spacing: -0.03em;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}

/* Dark mode toggle */
.dark-mode-toggle {
  flex-shrink: 0;
  width: 44px;
  height: 44px;
  display: grid;
  place-items: center;
  font-size: 1.25rem;
  line-height: 1;
  padding: 0;
  border: 1px solid var(--border);
  border-radius: var(--radius-pill);
  background: var(--surface-2);
  color: var(--text);
  cursor: pointer;
  transition: transform 0.2s var(--ease), background-color 0.2s var(--ease),
    border-color 0.2s var(--ease);
}

.dark-mode-toggle:hover {
  transform: rotate(-12deg) scale(1.08);
  border-color: var(--accent);
}

/* =========================================================================
   Form
   ========================================================================= */
#todo-form {
  margin-bottom: 1.75rem;
}

.input-row {
  display: flex;
  gap: 0.6rem;
}

#todo-input {
  flex: 1;
  min-width: 0;
  padding: 0.85rem 1.1rem;
  font-size: 1rem;
  font-family: inherit;
  color: var(--text);
  background: var(--surface-2);
  border: 1.5px solid transparent;
  border-radius: var(--radius-sm);
  transition: border-color 0.2s var(--ease), box-shadow 0.2s var(--ease),
    background-color 0.2s var(--ease);
}

#todo-input::placeholder {
  color: var(--muted);
}

#todo-input:focus {
  outline: none;
  background: var(--surface);
  border-color: var(--accent);
  box-shadow: var(--ring);
}

/* Add button */
#add-button {
  flex-shrink: 0;
  padding: 0.85rem 1.4rem;
  font-size: 0.95rem;
  font-weight: 600;
  font-family: inherit;
  color: #fff;
  white-space: nowrap;
  border: none;
  border-radius: var(--radius-sm);
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  box-shadow: 0 6px 16px -8px var(--accent);
  cursor: pointer;
  transition: transform 0.18s var(--ease), box-shadow 0.18s var(--ease),
    opacity 0.18s var(--ease);
}

#add-button:hover:not(:disabled) {
  transform: translateY(-2px);
  box-shadow: 0 12px 24px -10px var(--accent);
}

#add-button:active:not(:disabled) {
  transform: translateY(0);
}

#add-button:disabled {
  opacity: 0.45;
  cursor: not-allowed;
  box-shadow: none;
}

#char-count {
  margin-top: 0.5rem;
  text-align: right;
  font-size: 0.8rem;
  font-variant-numeric: tabular-nums;
  color: var(--muted);
}

/* =========================================================================
   Todo list
   ========================================================================= */
#todo-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

#todo-list li {
  position: relative;
  display: flex;
  align-items: center;
  gap: 1rem;
  width: 100%;
  padding: 0.9rem 1rem 0.9rem 2.25rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow-sm);
  cursor: grab;
  user-select: none;
  -webkit-user-select: none;
  transition: transform 0.2s var(--ease), box-shadow 0.2s var(--ease),
    border-color 0.2s var(--ease), background-color 0.2s var(--ease),
    opacity 0.2s var(--ease);
}

/* Drag handle */
#todo-list li::before {
  content: "⠿";
  position: absolute;
  left: 0.7rem;
  top: 50%;
  transform: translateY(-50%);
  color: var(--muted);
  font-size: 1rem;
  opacity: 0;
  transition: opacity 0.2s var(--ease);
}

#todo-list li:hover {
  transform: translateY(-2px);
  border-color: var(--border-strong);
  box-shadow: var(--shadow-md);
}

#todo-list li:hover::before {
  opacity: 0.7;
}

/* Todo text */
.todo-text {
  flex: 1;
  min-width: 0;
  font-size: 1rem;
  color: var(--text);
  word-break: break-word;
  overflow-wrap: anywhere;
  transition: color 0.2s var(--ease), opacity 0.2s var(--ease);
}

/* Completed state */
#todo-list li.completed {
  background: var(--surface-2);
  border-color: transparent;
}

#todo-list li.completed .todo-text {
  text-decoration: line-through;
  text-decoration-color: var(--success);
  color: var(--muted);
  opacity: 0.8;
}

/* Buttons group */
.todo-buttons {
  display: flex;
  gap: 0.45rem;
  flex-shrink: 0;
}

.todo-buttons button {
  padding: 0.5rem 0.95rem;
  font-size: 0.82rem;
  font-weight: 600;
  font-family: inherit;
  color: #fff;
  border: none;
  border-radius: var(--radius-pill);
  cursor: pointer;
  transition: transform 0.15s var(--ease), box-shadow 0.15s var(--ease),
    background-color 0.15s var(--ease), filter 0.15s var(--ease);
}

.todo-buttons button:hover {
  transform: translateY(-1px);
  filter: brightness(1.05);
}

.todo-buttons button:active {
  transform: translateY(0);
}

/* Complete / Undo (first button) */
.todo-buttons button:first-child {
  background: var(--success);
  box-shadow: 0 4px 12px -6px var(--success);
}

/* Pressed/active look once completed (aria-pressed reflects state) */
.todo-buttons button:first-child[aria-pressed="true"] {
  background: transparent;
  color: var(--success-strong);
  box-shadow: inset 0 0 0 1.5px var(--success);
}

/* Delete (last button) */
.todo-buttons button:last-child {
  background: var(--danger);
  box-shadow: 0 4px 12px -6px var(--danger);
}

/* =========================================================================
   Interaction states
   ========================================================================= */
.fade-in {
  animation: fadeIn 0.3s var(--ease) both;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: translateY(0); }
}

.dragging {
  opacity: 0.6;
  cursor: grabbing !important;
  box-shadow: var(--shadow-lg);
  transform: scale(1.02);
}

.drag-over {
  border-top: 2px solid var(--accent) !important;
}

.disable-selection {
  user-select: none !important;
  -webkit-user-select: none !important;
}

/* =========================================================================
   Focus visibility (accessibility)
   ========================================================================= */
#todo-list li:focus-visible,
button:focus-visible,
#todo-input:focus-visible {
  outline: none;
  box-shadow: var(--ring);
}

#todo-list li:focus-visible {
  border-color: var(--accent);
}

/* =========================================================================
   Responsive
   ========================================================================= */
@media (max-width: 640px) {
  .input-row {
    flex-direction: column;
  }

  #add-button {
    width: 100%;
  }

  #todo-list li {
    flex-direction: column;
    align-items: stretch;
    gap: 0.75rem;
    padding-left: 1rem;
  }

  #todo-list li::before {
    display: none;
  }

  .todo-buttons {
    width: 100%;
  }

  .todo-buttons button {
    flex: 1;
  }
}

/* =========================================================================
   Reduced motion
   ========================================================================= */
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.001ms !important;
  }
}