feat: add bingo ball spinning sound effect

- Add `sfx-bingo-balls` audio asset and load it in PreloadScene
- Register `BINGO_BALLS` sound in the SFX constants
- Play the sound when the bingo ball spinning animation starts

public/src/games/bingo/BingoGame.js

@@ -533,6 +533,7 @@ export default class BingoGame extends Phaser.Scene {
     this.animating = true;
     this.nextBtn.setEnabled(false);
     this.countText.setText('Spinning…');
+    playSound(this, SFX.BINGO_BALLS);
     // Spin up to full speed (~2s), hold there for 5s, then draw a ball.
     this.setSpin(1, 2000, 'Quad.easeIn', () => {
       this.spinHoldTimer = this.time.delayedCall(5000, () => {

public/src/scenes/PreloadScene.js

@@ -56,6 +56,7 @@ export default class PreloadScene extends Phaser.Scene {
     this.load.audio('sfx-casino-win',       '/assets/fx/casino-win.mp3');
     this.load.audio('sfx-chip-bet',         '/assets/fx/chip-bet.mp3');
     this.load.audio('sfx-dice-roll',        '/assets/fx/dice-roll.mp3');
+    this.load.audio('sfx-bingo-balls',      '/assets/fx/bingo-balls.mp3');
     this.load.audio('sfx-pencil-write',     '/assets/fx/pencil-write.mp3');
     this.load.audio('sfx-piece-click',      '/assets/fx/piece-click.mp3');
     this.load.audio('sfx-roulette',         '/assets/fx/roulette.mp3');

public/src/ui/Sounds.js

@@ -10,6 +10,7 @@ export const SFX = {
   CHIP_BET:         'sfx-chip-bet',
   DICE_ROLL:        'sfx-dice-roll',
   PENCIL_WRITE:     'sfx-pencil-write',
+  BINGO_BALLS:      'sfx-bingo-balls',
   PIECE_CLICK:      'sfx-piece-click',
   ROULETTE:         'sfx-roulette',
 };