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/TransitionScene.js

@@ -1,5 +1,6 @@
 import Phaser from 'phaser';
 import { fontSize } from '../utils/responsive.js';
+import { createFullscreenButton } from '../utils/fullscreen.js';
 
 export default class TransitionScene extends Phaser.Scene {
     constructor() {
@@ -100,6 +101,9 @@ export default class TransitionScene extends Phaser.Scene {
             // Proceed to next scene
             this.scene.start(this.nextScene, { inventory: this.inventory });
         });
+
+        // Fullscreen button
+        createFullscreenButton(this);
     }
 
     getDestinationContent(destination) {