:root {
  --bg: #0f172a;
  --surface: #1e293b;
  --cell: #152036;
  --text: #e2e8f0;
  --muted: #94a3b8;
  --accent: #f97316;
  --accent-hover: #fb8a3c;
  --on-accent: #1a0d03;
  --line: rgba(148, 163, 184, 0.22);
  --radius: 12px;
  --shadow: 0 12px 32px rgba(2, 6, 23, 0.5);
  --ease: cubic-bezier(0.2, 0.8, 0.2, 1);
  color-scheme: dark;
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
}

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

html {
  -webkit-text-size-adjust: 100%;
  text-size-adjust: 100%;
}

body {
  margin: 0;
  min-height: 100svh;
  overflow-x: hidden;
  background:
    radial-gradient(900px 520px at 85% -10%, rgba(249, 115, 22, 0.13), transparent 65%),
    radial-gradient(700px 480px at -10% 110%, rgba(56, 189, 248, 0.08), transparent 65%),
    var(--bg);
  color: var(--text);
  line-height: 1.5;
}

.app {
  display: flex;
  flex-direction: column;
  width: min(100% - 32px, 480px);
  min-height: 100svh;
  margin-inline: auto;
}

/* ---------- Header ---------- */

.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  padding: 16px 0 12px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  margin: 0;
  font-size: 2.25rem;
  font-weight: 800;
  line-height: 1;
  letter-spacing: -0.03em;
  color: var(--accent);
}

.brand-mark {
  width: 28px;
  height: 28px;
  border-radius: 9px;
  background:
    linear-gradient(var(--bg), var(--bg)) 50% 50% / 2px 100% no-repeat,
    linear-gradient(var(--bg), var(--bg)) 50% 50% / 100% 2px no-repeat,
    linear-gradient(135deg, var(--accent), #8b5cf6);
  box-shadow: 0 4px 14px rgba(249, 115, 22, 0.35);
}

.topbar-actions {
  display: flex;
  gap: 8px;
}

/* ---------- Buttons ---------- */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  min-width: 44px;
  min-height: 44px;
  padding: 0 16px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--text);
  font: inherit;
  font-size: 0.95rem;
  font-weight: 650;
  cursor: pointer;
  transition:
    background-color 180ms var(--ease),
    border-color 180ms var(--ease),
    opacity 180ms var(--ease),
    transform 150ms var(--ease);
}

.btn:hover:not(:disabled) {
  border-color: rgba(249, 115, 22, 0.65);
}

.btn:active:not(:disabled) {
  transform: translateY(1px) scale(0.98);
}

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

.btn:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

.btn-primary {
  border-color: var(--accent);
  background: var(--accent);
  color: var(--on-accent);
  box-shadow: 0 6px 18px rgba(249, 115, 22, 0.28);
}

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

.btn-lang {
  min-width: 48px;
  padding: 0 12px;
  font-weight: 800;
  letter-spacing: 0.06em;
}

.btn-undo {
  flex: none;
}

.icon {
  flex: none;
}

/* ---------- Score HUD ---------- */

.layout {
  display: flex;
  flex: 1;
  flex-direction: column;
  gap: 16px;
}

.tagline {
  margin: 0 0 12px;
  color: var(--muted);
  font-size: 0.95rem;
}

.stats {
  display: flex;
  align-items: stretch;
  gap: 8px;
}

.stat {
  position: relative;
  display: flex;
  flex: 1;
  flex-direction: column;
  justify-content: center;
  min-width: 0;
  padding: 6px 14px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
}

.stat-label {
  color: var(--muted);
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.stat-value {
  font-size: 1.5rem;
  font-weight: 800;
  line-height: 1.2;
  font-variant-numeric: tabular-nums;
}

.score-float {
  position: absolute;
  top: 6px;
  right: 12px;
  color: var(--accent);
  font-weight: 800;
  pointer-events: none;
  animation: score-float 800ms var(--ease) forwards;
}

/* ---------- Board ---------- */

.board-area {
  display: flex;
  justify-content: center;
}

.board-wrap {
  position: relative;
  width: min(100%, 480px);
  container-type: inline-size;
}

.board {
  --gap: 2.8cqi;
  position: relative;
  aspect-ratio: 1;
  padding: var(--gap);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: var(--shadow), inset 0 0 0 1px var(--line);
  touch-action: none;
  user-select: none;
  -webkit-user-select: none;
  -webkit-tap-highlight-color: transparent;
}

.board-grid,
.board-tiles {
  position: absolute;
  inset: var(--gap);
}

.board-grid {
  display: grid;
  grid-template-columns: repeat(var(--size), 1fr);
  grid-template-rows: repeat(var(--size), 1fr);
  gap: var(--gap);
}

.cell {
  border-radius: 8px;
  background: var(--cell);
  box-shadow: inset 0 2px 6px rgba(2, 6, 23, 0.35);
}

.tile {
  --tile-bg: #475569;
  --tile-fg: #f8fafc;
  --tile-glow: transparent;
  position: absolute;
  top: 0;
  left: 0;
  z-index: 1;
  width: calc((100% - (var(--size) - 1) * var(--gap)) / var(--size));
  height: calc((100% - (var(--size) - 1) * var(--gap)) / var(--size));
  transform: translate(calc(var(--col) * (100% + var(--gap))), calc(var(--row) * (100% + var(--gap))));
  transition: transform 120ms ease-out;
}

.tile-merged {
  z-index: 2;
}

.tile-leaving {
  z-index: 0;
}

.tile-inner {
  display: grid;
  place-items: center;
  width: 100%;
  height: 100%;
  border-radius: 8px;
  background: var(--tile-bg);
  color: var(--tile-fg);
  font-size: 10cqi;
  font-weight: 800;
  line-height: 1;
  letter-spacing: -0.02em;
  font-variant-numeric: tabular-nums;
  box-shadow:
    0 0 22px var(--tile-glow),
    0 3px 0 rgba(2, 6, 23, 0.35),
    inset 0 1px 0 rgba(255, 255, 255, 0.14);
}

.tile[data-digits="3"] .tile-inner { font-size: 8.4cqi; }
.tile[data-digits="4"] .tile-inner { font-size: 6.6cqi; }
.tile[data-digits="5"] .tile-inner { font-size: 5.2cqi; }

.tile-new .tile-inner {
  animation: tile-appear 200ms var(--ease) 110ms backwards;
}

.tile-merged .tile-inner {
  animation: tile-pop 200ms var(--ease) 100ms backwards;
}

/* Original palette: cool blues and violets that warm up into the orange accent. */
.tile[data-value="2"]    { --tile-bg: #2a4366; --tile-fg: #dbeafe; }
.tile[data-value="4"]    { --tile-bg: #2557a0; --tile-fg: #e0f2fe; }
.tile[data-value="8"]    { --tile-bg: #0e7a99; --tile-fg: #ecfeff; }
.tile[data-value="16"]   { --tile-bg: #0f9488; --tile-fg: #f0fdfa; }
.tile[data-value="32"]   { --tile-bg: #4f46e5; --tile-fg: #eef2ff; }
.tile[data-value="64"]   { --tile-bg: #7c3aed; --tile-fg: #f5f3ff; }
.tile[data-value="128"]  { --tile-bg: #b0219f; --tile-fg: #fdf4ff; --tile-glow: rgba(192, 38, 211, 0.25); }
.tile[data-value="256"]  { --tile-bg: #d61f5c; --tile-fg: #fff1f2; --tile-glow: rgba(225, 29, 72, 0.3); }
.tile[data-value="512"]  { --tile-bg: #e4500c; --tile-fg: #fff7ed; --tile-glow: rgba(234, 88, 12, 0.35); }
.tile[data-value="1024"] { --tile-bg: #f97316; --tile-fg: #1a0d03; --tile-glow: rgba(249, 115, 22, 0.45); }
.tile[data-value="2048"] { --tile-bg: linear-gradient(135deg, #fb923c, #facc15); --tile-fg: #1a1203; --tile-glow: rgba(250, 204, 21, 0.55); }
/* Past 2048 ([data-super]) the text turns white and each value gets its own gradient, up to the largest possible tile. */
.tile[data-super]          { --tile-fg: #ffffff; }
.tile[data-value="4096"]   { --tile-bg: linear-gradient(135deg, #f97316, #e11d48); --tile-glow: rgba(244, 63, 94, 0.45); }
.tile[data-value="8192"]   { --tile-bg: linear-gradient(135deg, #e11d48, #9333ea); --tile-glow: rgba(168, 85, 247, 0.45); }
.tile[data-value="16384"]  { --tile-bg: linear-gradient(135deg, #7c3aed, #2563eb); --tile-glow: rgba(99, 102, 241, 0.5); }
.tile[data-value="32768"]  { --tile-bg: linear-gradient(135deg, #2563eb, #0891b2); --tile-glow: rgba(6, 182, 212, 0.45); }
.tile[data-value="65536"]  { --tile-bg: linear-gradient(135deg, #0891b2, #059669); --tile-glow: rgba(16, 185, 129, 0.45); }
.tile[data-value="131072"] { --tile-bg: linear-gradient(135deg, #f97316, #db2777 35%, #7c3aed 65%, #0891b2); --tile-glow: rgba(249, 115, 22, 0.55); }

/* ---------- Overlays ---------- */

.overlay {
  position: absolute;
  inset: 0;
  z-index: 5;
  display: grid;
  place-content: center;
  justify-items: center;
  gap: 12px;
  padding: 24px;
  border-radius: var(--radius);
  background: rgba(15, 23, 42, 0.8);
  backdrop-filter: blur(3px);
  text-align: center;
  animation: overlay-in 250ms var(--ease) 250ms backwards;
}

.overlay[hidden] {
  display: none;
}

.overlay-title {
  margin: 0;
  font-size: clamp(1.6rem, 8cqi, 2.4rem);
  line-height: 1.15;
}

.overlay-win .overlay-title {
  color: var(--accent);
}

.overlay-text {
  max-width: 30ch;
  margin: 0;
  color: var(--muted);
}

.overlay-actions {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 8px;
  margin-top: 4px;
}

/* ---------- Help + footer ---------- */

.panel-help {
  padding: 14px 18px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: rgba(30, 41, 59, 0.6);
}

.panel-help h2 {
  margin: 0 0 6px;
  font-size: 1rem;
}

.panel-help ul {
  display: grid;
  gap: 4px;
  margin: 0;
  padding-left: 1.1em;
  color: var(--muted);
  font-size: 0.92rem;
}

.panel-help li::marker {
  color: var(--accent);
}

.footer {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  padding: 16px 0 20px;
  color: var(--muted);
  font-size: 0.9rem;
}

.footer a {
  display: inline-flex;
  align-items: center;
  min-height: 40px;
  padding: 0 4px;
  border-radius: 8px;
  color: var(--accent);
  text-decoration: none;
  transition: color 150ms var(--ease);
}

.footer a:hover {
  color: var(--accent-hover);
  text-decoration: underline;
}

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

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

/* ---------- Wide screens: stats and help beside the board ---------- */

@media (min-width: 880px) {
  .app {
    width: min(100% - 48px, 920px);
  }

  .topbar {
    padding: 28px 0 24px;
  }

  .layout {
    display: grid;
    grid-template-columns: minmax(0, 1fr) auto;
    grid-template-rows: auto 1fr;
    grid-template-areas:
      "stats board"
      "help board";
    gap: 20px 40px;
    align-content: start;
  }

  .panel-stats { grid-area: stats; }
  .board-area { grid-area: board; }
  .panel-help { grid-area: help; align-self: start; }

  .tagline {
    margin-bottom: 16px;
    font-size: 1.05rem;
  }

  .board-wrap {
    width: clamp(320px, 100svh - 230px, 480px);
  }
}

/* ---------- Animations ---------- */

@keyframes tile-appear {
  from { opacity: 0; transform: scale(0.4); }
  to { opacity: 1; transform: scale(1); }
}

@keyframes tile-pop {
  0% { opacity: 0; transform: scale(0.6); }
  55% { opacity: 1; transform: scale(1.14); }
  100% { transform: scale(1); }
}

@keyframes score-float {
  from { opacity: 1; transform: translateY(0); }
  to { opacity: 0; transform: translateY(-28px); }
}

@keyframes overlay-in {
  from { opacity: 0; }
  to { opacity: 1; }
}

@media (prefers-reduced-motion: reduce) {
  .tile,
  .btn {
    transition: none;
  }

  .tile-inner,
  .overlay,
  .score-float {
    animation: none;
  }
}