:root {
  color-scheme: dark;
  --bg: #0f172a;
  --panel: rgba(15, 23, 42, 0.76);
  --panel-strong: rgba(30, 41, 59, 0.92);
  --text: #f8fafc;
  --muted: #cbd5e1;
  --accent: #38bdf8;
  --accent-2: #a78bfa;
  --cell: rgba(148, 163, 184, 0.18);
  --tile-text-dark: #1e293b;
  --tile-text-light: #fff7ed;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  min-height: 100vh;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  color: var(--text);
  background:
    radial-gradient(circle at 18% 12%, rgba(56, 189, 248, 0.28), transparent 34%),
    radial-gradient(circle at 82% 18%, rgba(167, 139, 250, 0.25), transparent 32%),
    linear-gradient(135deg, #020617 0%, #0f172a 54%, #111827 100%);
  display: grid;
  place-items: center;
  padding: 22px;
}

.shell {
  width: min(94vw, 560px);
}

.hero {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  align-items: end;
  margin-bottom: 18px;
}

.eyebrow {
  color: var(--accent);
  font-size: 0.78rem;
  font-weight: 800;
  letter-spacing: .15em;
  margin: 0 0 4px;
  text-transform: uppercase;
}

h1 {
  font-size: clamp(3.4rem, 12vw, 5.6rem);
  line-height: .85;
  margin: 0;
  letter-spacing: -0.08em;
}

.subtitle {
  color: var(--muted);
  margin: 12px 0 0;
  max-width: 330px;
}

.scoreboard {
  display: grid;
  grid-template-columns: repeat(2, minmax(86px, 1fr));
  gap: 10px;
}

.score-card {
  background: var(--panel);
  border: 1px solid rgba(255,255,255,.12);
  border-radius: 18px;
  padding: 12px 14px;
  text-align: center;
  box-shadow: 0 20px 60px rgba(0,0,0,.28);
  backdrop-filter: blur(14px);
}

.score-card span {
  color: #94a3b8;
  display: block;
  font-size: .72rem;
  font-weight: 800;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.score-card strong {
  display: block;
  font-size: 1.45rem;
  margin-top: 4px;
}

.toolbar {
  display: flex;
  gap: 10px;
  margin-bottom: 14px;
}

button {
  border: 0;
  border-radius: 14px;
  color: white;
  background: linear-gradient(135deg, #0284c7, #7c3aed);
  font-weight: 800;
  padding: 12px 16px;
  cursor: pointer;
  box-shadow: 0 16px 32px rgba(2, 132, 199, .25);
  transition: transform .14s ease, filter .14s ease;
}

button:hover { transform: translateY(-1px); filter: brightness(1.06); }
button:active { transform: translateY(1px); }

#undo {
  background: rgba(148, 163, 184, .22);
  border: 1px solid rgba(255,255,255,.14);
  box-shadow: none;
}

.board-wrap {
  position: relative;
  touch-action: none;
}

.board {
  --gap: clamp(9px, 2vw, 14px);
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--gap);
  background: var(--panel-strong);
  border: 1px solid rgba(255,255,255,.14);
  border-radius: 26px;
  padding: var(--gap);
  aspect-ratio: 1;
  box-shadow: 0 28px 80px rgba(0,0,0,.36), inset 0 1px 0 rgba(255,255,255,.08);
  backdrop-filter: blur(18px);
}

.cell {
  display: grid;
  place-items: center;
  border-radius: 18px;
  background: var(--cell);
  font-size: clamp(1.45rem, 7vw, 2.9rem);
  font-weight: 950;
  line-height: 1;
  user-select: none;
  transition: transform .12s ease, background .16s ease, color .16s ease;
}

.cell:not(:empty) {
  transform: scale(1.01);
  box-shadow: inset 0 -5px 0 rgba(0,0,0,.08), 0 12px 22px rgba(0,0,0,.16);
}

.v2 { background: #e0f2fe; color: var(--tile-text-dark); }
.v4 { background: #bae6fd; color: var(--tile-text-dark); }
.v8 { background: #7dd3fc; color: var(--tile-text-dark); }
.v16 { background: #38bdf8; color: var(--tile-text-dark); }
.v32 { background: #22d3ee; color: var(--tile-text-dark); }
.v64 { background: #2dd4bf; color: var(--tile-text-dark); }
.v128 { background: #a7f3d0; color: var(--tile-text-dark); font-size: clamp(1.2rem, 5.8vw, 2.25rem); }
.v256 { background: #fde68a; color: var(--tile-text-dark); font-size: clamp(1.2rem, 5.8vw, 2.25rem); }
.v512 { background: #fdba74; color: var(--tile-text-dark); font-size: clamp(1.2rem, 5.8vw, 2.25rem); }
.v1024 { background: #fb7185; color: var(--tile-text-light); font-size: clamp(1rem, 5vw, 2rem); }
.v2048 { background: linear-gradient(135deg, #f59e0b, #ec4899, #8b5cf6); color: var(--tile-text-light); font-size: clamp(1rem, 5vw, 2rem); }
.v4096, .v8192 { background: #111827; color: var(--tile-text-light); font-size: clamp(.92rem, 4.6vw, 1.75rem); }

.overlay {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  background: rgba(2, 6, 23, .72);
  border-radius: 26px;
  backdrop-filter: blur(8px);
}

.overlay.hidden { display: none; }

.modal {
  width: min(84%, 330px);
  text-align: center;
  background: rgba(15, 23, 42, .92);
  border: 1px solid rgba(255,255,255,.14);
  border-radius: 22px;
  padding: 24px;
}

.modal h2 { margin: 0 0 8px; font-size: 2rem; }
.modal p { color: var(--muted); margin: 0 0 18px; }
.hint { color: #94a3b8; text-align: center; }

@media (max-width: 560px) {
  body { padding: 14px; align-items: start; }
  .hero { display: block; }
  .scoreboard { margin-top: 16px; }
}