Sprint 3: architecture — UI utility, achievements menu

- Extract createButton/pixelText helpers to src/utils/ui.js with
  sensible defaults and per-call option overrides
- MenuScene now shows BADGES button opening the achievement panel
  (10 entries, count of unlocked, star icons for completed)
- GameOverScene buttons migrated to shared utility, removing duplicate
  hover/click handlers
- Smaller LEADERBOARD button to make room for BADGES alongside

src/utils/ui.js

@@ -0,0 +1,51 @@
+import { sound } from '../managers/SoundManager.js';
+
+const DEFAULTS = {
+  width: 260,
+  height: 48,
+  fontSize: '13px',
+  bgColor: 0x581c87,
+  hoverColor: 0x7e22ce,
+  strokeColor: 0xa855f7,
+  strokeWidth: 2,
+  textColor: '#ffffff',
+  hoverScale: 1.04,
+  playClick: true,
+};
+
+export function createButton(scene, x, y, text, callback, options = {}) {
+  const opt = { ...DEFAULTS, ...options };
+
+  const bg = scene.add.rectangle(x, y, opt.width, opt.height, opt.bgColor)
+    .setStrokeStyle(opt.strokeWidth, opt.strokeColor)
+    .setInteractive({ useHandCursor: true });
+
+  const label = scene.add.text(x, y, text, {
+    fontFamily: '"Press Start 2P", monospace',
+    fontSize: opt.fontSize,
+    color: opt.textColor,
+  }).setOrigin(0.5);
+
+  bg.on('pointerover', () => {
+    bg.setFillStyle(opt.hoverColor);
+    scene.tweens.add({ targets: [bg, label], scaleX: opt.hoverScale, scaleY: opt.hoverScale, duration: 100 });
+  });
+  bg.on('pointerout', () => {
+    bg.setFillStyle(opt.bgColor);
+    scene.tweens.add({ targets: [bg, label], scaleX: 1, scaleY: 1, duration: 100 });
+  });
+  bg.on('pointerdown', () => {
+    if (opt.playClick) sound.click();
+    callback();
+  });
+
+  return { bg, label, destroy: () => { bg.destroy(); label.destroy(); } };
+}
+
+export function pixelText(scene, x, y, text, size = '14px', color = '#ffffff') {
+  return scene.add.text(x, y, text, {
+    fontFamily: '"Press Start 2P", monospace',
+    fontSize: size,
+    color,
+  }).setOrigin(0.5);
+}