/* ===========================
   Design Tokens
   =========================== */
:root {
  --bg-primary: #0D1117;
  --bg-secondary: #161B22;
  --bg-tertiary: #21262D;
  --text-primary: #F0F6FC;
  --text-secondary: #8B949E;
  --text-muted: #484F58;
  --accent: #3B82F6;
  --accent-glow: rgba(59, 130, 246, 0.3);
  --border: #30363D;
  --btn-bg: #21262D;
  --btn-active: #30363D;
  --overlay-bg: rgba(0, 0, 0, 0.85);
  --board-bg: #0A0E14;

  --cell-gap: 10px;
  --cell-size: calc((min(100vw, 480px) - 48px - var(--cell-gap) * 3) / 4);
  --board-size: calc(var(--cell-size) * 4 + var(--cell-gap) * 3 + 24px);
  --radius: 10px;
  --tile-radius: 8px;
}

@media (prefers-color-scheme: light) {
  :root {
    --bg-primary: #FFFFFF;
    --bg-secondary: #F6F8FA;
    --bg-tertiary: #E9ECF0;
    --text-primary: #1B1E28;
    --text-secondary: #57606A;
    --text-muted: #8C959F;
    --accent: #3182F6;
    --accent-glow: rgba(49, 130, 246, 0.2);
    --border: #D0D7DE;
    --btn-bg: #E9ECF0;
    --btn-active: #D0D7DE;
    --overlay-bg: rgba(255, 255, 255, 0.9);
    --board-bg: #F0F2F5;
  }
}

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

html, body {
  height: 100%;
  width: 100%;
  overflow: hidden;
  touch-action: none;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Pretendard', 'Segoe UI', Roboto, sans-serif;
  background: var(--bg-primary);
  color: var(--text-primary);
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100dvh;
  -webkit-tap-highlight-color: transparent;
  user-select: none;
}

/* ===========================
   Overlay Screens
   =========================== */
.overlay {
  position: fixed;
  inset: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--overlay-bg);
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease;
}

.overlay.active {
  opacity: 1;
  pointer-events: all;
}

.overlay-content {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 20px;
  padding: 40px 32px;
  max-width: 360px;
  width: 90%;
  background: var(--bg-secondary);
  border: 1px solid var(--border);
  border-radius: 20px;
  box-shadow: 0 20px 60px rgba(0, 0, 0, 0.4);
}

/* ===========================
   Game Title (gradient)
   =========================== */
.game-title {
  font-size: 72px;
  font-weight: 900;
  letter-spacing: -2px;
  background: linear-gradient(135deg, #3B82F6, #8B5CF6, #EC4899);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  line-height: 1;
}

.game-desc {
  font-size: 16px;
  color: var(--text-secondary);
  text-align: center;
  line-height: 1.6;
}

.game-hint {
  font-size: 13px;
  color: var(--text-muted);
  letter-spacing: 0.5px;
}

/* ===========================
   CTA Button
   =========================== */
.cta-btn {
  padding: 14px 48px;
  font-size: 16px;
  font-weight: 700;
  font-family: inherit;
  color: #fff;
  background: linear-gradient(135deg, #3B82F6, #8B5CF6);
  border: none;
  border-radius: 50px;
  cursor: pointer;
  transition: transform 0.15s ease, box-shadow 0.15s ease;
  box-shadow: 0 4px 20px var(--accent-glow);
  letter-spacing: 0.5px;
}

.cta-btn:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 28px var(--accent-glow);
}

.cta-btn:active {
  transform: translateY(0);
  box-shadow: 0 2px 10px var(--accent-glow);
}

/* ===========================
   Game Over Overlay specifics
   =========================== */
.over-title {
  font-size: 32px;
  font-weight: 800;
  color: var(--text-primary);
}

.new-record {
  font-size: 15px;
  font-weight: 700;
  color: #F59E0B;
  background: rgba(245, 158, 11, 0.12);
  border: 1px solid rgba(245, 158, 11, 0.3);
  padding: 6px 18px;
  border-radius: 50px;
}

.new-record.hidden {
  display: none;
}

.score-summary {
  display: flex;
  flex-direction: column;
  gap: 12px;
  width: 100%;
}

.score-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 12px 16px;
  background: var(--bg-tertiary);
  border-radius: var(--radius);
  border: 1px solid var(--border);
}

.score-row .score-label {
  font-size: 14px;
  color: var(--text-secondary);
  font-weight: 500;
}

.score-row .score-val {
  font-size: 22px;
  font-weight: 800;
  color: var(--text-primary);
}

.score-row .score-val.accent {
  color: var(--accent);
}

/* ===========================
   Game Wrapper
   =========================== */
#game-wrapper {
  display: flex;
  flex-direction: column;
  align-items: center;
  width: 100%;
  max-width: 480px;
  height: 100dvh;
  padding: 0 16px;
  padding-top: env(safe-area-inset-top);
  padding-bottom: env(safe-area-inset-bottom);
  gap: 0;
}

/* ===========================
   Header
   =========================== */
.game-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: 100%;
  padding: 20px 0 12px;
}

.header-title {
  font-size: 36px;
  font-weight: 900;
  letter-spacing: -1px;
  background: linear-gradient(135deg, #3B82F6, #8B5CF6, #EC4899);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.score-area {
  display: flex;
  gap: 8px;
}

.score-box {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 8px 16px;
  background: var(--bg-secondary);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  min-width: 70px;
}

.score-box .score-label {
  font-size: 11px;
  color: var(--text-muted);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.5px;
}

.score-number {
  font-size: 20px;
  font-weight: 800;
  color: var(--text-primary);
  line-height: 1.2;
  transition: transform 0.15s ease;
}

/* ===========================
   Controls
   =========================== */
.controls {
  display: flex;
  justify-content: flex-end;
  width: 100%;
  padding: 0 0 12px;
}

.control-btn {
  padding: 8px 20px;
  font-size: 13px;
  font-weight: 600;
  font-family: inherit;
  color: var(--text-secondary);
  background: var(--btn-bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  cursor: pointer;
  transition: background 0.15s ease, color 0.15s ease, border-color 0.15s ease;
  letter-spacing: 0.3px;
}

.control-btn:hover {
  background: var(--btn-active);
  color: var(--text-primary);
  border-color: var(--accent);
}

.control-btn:active {
  transform: scale(0.97);
}

/* ===========================
   Board
   =========================== */
.board-wrapper {
  display: flex;
  align-items: center;
  justify-content: center;
  flex: 1;
  width: 100%;
}

.board {
  position: relative;
  width: var(--board-size);
  height: var(--board-size);
  background: var(--board-bg);
  border-radius: 14px;
  padding: 12px;
  border: 1px solid var(--border);
  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.3), 0 0 0 1px var(--border);
}

/* ===========================
   Grid Background Cells
   =========================== */
.grid-bg {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--cell-gap);
  width: 100%;
  height: 100%;
}

.cell {
  background: var(--bg-tertiary);
  border-radius: var(--tile-radius);
  opacity: 0.5;
}

/* ===========================
   Tile Container
   =========================== */
.tile-container {
  position: absolute;
  top: 12px;
  left: 12px;
  right: 12px;
  bottom: 12px;
}

/* ===========================
   Tiles
   =========================== */
.tile {
  position: absolute;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--tile-radius);
  font-weight: 800;
  transition: left 0.12s ease, top 0.12s ease;
  will-change: left, top, transform;
}

.tile.tile-new {
  animation: tileAppear 0.2s ease forwards;
}

.tile.tile-merge {
  animation: tileMerge 0.2s ease forwards;
}

@keyframes tileAppear {
  0%   { transform: scale(0);   opacity: 0; }
  70%  { transform: scale(1.08); opacity: 1; }
  100% { transform: scale(1);   opacity: 1; }
}

@keyframes tileMerge {
  0%   { transform: scale(1); }
  40%  { transform: scale(1.18); }
  100% { transform: scale(1); }
}

/* ===========================
   Tile Color Palette
   =========================== */
.tile[data-value="2"]    { background: #1E3A5F; color: #9ECBFF; }
.tile[data-value="4"]    { background: #1B3A2F; color: #7EE8A2; }
.tile[data-value="8"]    { background: #2D2A1A; color: #F1C232; }
.tile[data-value="16"]   { background: #2D1F10; color: #FFB347; }
.tile[data-value="32"]   { background: #3A1A12; color: #FF7F50; }
.tile[data-value="64"]   { background: #3D0E0E; color: #FF5252; }
.tile[data-value="128"]  { background: #2D1A40; color: #CE93D8; }
.tile[data-value="256"]  { background: #1A1F4D; color: #82B1FF; }
.tile[data-value="512"]  { background: #00274D; color: #40C4FF; }
.tile[data-value="1024"] { background: #00332A; color: #64FFDA; }
.tile[data-value="2048"] {
  background: linear-gradient(135deg, #3B82F6, #8B5CF6, #EC4899);
  color: #FFFFFF;
  box-shadow: 0 0 24px rgba(139, 92, 246, 0.6), 0 0 48px rgba(59, 130, 246, 0.3);
}
.tile[data-value="4096"] {
  background: linear-gradient(135deg, #EC4899, #F97316, #EAB308);
  color: #FFFFFF;
  box-shadow: 0 0 24px rgba(236, 72, 153, 0.6);
}

/* Light mode tile overrides */
@media (prefers-color-scheme: light) {
  .tile[data-value="2"]    { background: #DBEAFE; color: #1D4ED8; }
  .tile[data-value="4"]    { background: #D1FAE5; color: #065F46; }
  .tile[data-value="8"]    { background: #FEF9C3; color: #854D0E; }
  .tile[data-value="16"]   { background: #FEF3C7; color: #92400E; }
  .tile[data-value="32"]   { background: #FFEDD5; color: #9A3412; }
  .tile[data-value="64"]   { background: #FEE2E2; color: #991B1B; }
  .tile[data-value="128"]  { background: #F3E8FF; color: #6B21A8; }
  .tile[data-value="256"]  { background: #EFF6FF; color: #1E40AF; }
  .tile[data-value="512"]  { background: #E0F2FE; color: #0C4A6E; }
  .tile[data-value="1024"] { background: #CCFBF1; color: #134E4A; }
  .tile[data-value="2048"] {
    background: linear-gradient(135deg, #3B82F6, #8B5CF6, #EC4899);
    color: #FFFFFF;
  }
}

/* ===========================
   Footer
   =========================== */
.game-footer {
  padding: 12px 0 16px;
  text-align: center;
}

.game-footer p {
  font-size: 12px;
  color: var(--text-muted);
  letter-spacing: 0.3px;
}

/* ===========================
   Score Pop Animation
   =========================== */
@keyframes scorePop {
  0%   { transform: scale(1); }
  50%  { transform: scale(1.3); color: var(--accent); }
  100% { transform: scale(1); }
}

.score-pop {
  animation: scorePop 0.25s ease;
}