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/HuntingScene.js

@@ -1,5 +1,6 @@
 import Phaser from 'phaser';
 import { fontSize } from '../utils/responsive.js';
+import { createFullscreenButton } from '../utils/fullscreen.js';
 
 export default class HuntingScene extends Phaser.Scene {
     constructor() {
@@ -47,6 +48,9 @@ export default class HuntingScene extends Phaser.Scene {
         // Message display
         const shootMessage = this.isMobile ? 'Hunt whales! Tap to shoot harpoon.' : 'Hunt whales! Click or press SPACE to shoot harpoon.';
         this.showMessage(shootMessage);
+
+        // Fullscreen button (positioned to avoid HUD)
+        createFullscreenButton(this, { x: 30, y: 580 });
     }
 
     update() {