feat: add fullscreen mode toggle button

- Add src/utils/fullscreen.js with createFullscreenButton() helper
- Fullscreen button appears in bottom-right corner of all scenes
- Click to toggle fullscreen mode
- Uses Phaser's built-in scale manager

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/scenes/IntroScene.js

@@ -1,4 +1,5 @@
 import { fontSize } from '../utils/responsive.js';
+import { createFullscreenButton } from '../utils/fullscreen.js';
 
 export default class IntroScene extends Phaser.Scene {
     constructor() {
@@ -78,6 +79,9 @@ export default class IntroScene extends Phaser.Scene {
             fontSize: fontSize(16),
             fill: '#ffff99'
         }).setOrigin(0.5);
+
+        // Fullscreen button
+        createFullscreenButton(this);
     }
 
     drawWaves() {