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/utils/fullscreen.js

@@ -0,0 +1,75 @@
+// Fullscreen toggle utility for Phaser scenes
+
+import { fontSize } from './responsive.js';
+
+/**
+ * Create a fullscreen toggle button in the corner of the screen
+ * @param {Phaser.Scene} scene - The Phaser scene to add the button to
+ * @param {object} options - Optional configuration
+ * @param {number} options.x - X position (default: 770)
+ * @param {number} options.y - Y position (default: 580)
+ */
+export function createFullscreenButton(scene, options = {}) {
+    const x = options.x || 770;
+    const y = options.y || 580;
+
+    // Button background
+    const btn = scene.add.rectangle(x, y, 50, 30, 0x000000, 0.6);
+    btn.setStrokeStyle(2, 0xffffff);
+    btn.setInteractive({ useHandCursor: true });
+    btn.setScrollFactor(0);
+    btn.setDepth(1000);
+
+    // Button icon/text
+    const icon = scene.add.text(x, y, '⛶', {
+        fontSize: fontSize(18),
+        fill: '#ffffff'
+    }).setOrigin(0.5);
+    icon.setScrollFactor(0);
+    icon.setDepth(1001);
+
+    // Update icon based on fullscreen state
+    const updateIcon = () => {
+        if (scene.scale.isFullscreen) {
+            icon.setText('⛶');
+        } else {
+            icon.setText('⛶');
+        }
+    };
+
+    // Toggle fullscreen on click
+    btn.on('pointerdown', () => {
+        btn.setFillStyle(0x333333, 0.8);
+        btn.setScale(0.95);
+        icon.setScale(0.95);
+    });
+
+    btn.on('pointerup', () => {
+        btn.setFillStyle(0x000000, 0.6);
+        btn.setScale(1);
+        icon.setScale(1);
+
+        if (scene.scale.isFullscreen) {
+            scene.scale.stopFullscreen();
+        } else {
+            scene.scale.startFullscreen();
+        }
+        updateIcon();
+    });
+
+    btn.on('pointerover', () => {
+        btn.setFillStyle(0x333333, 0.8);
+    });
+
+    btn.on('pointerout', () => {
+        btn.setFillStyle(0x000000, 0.6);
+    });
+
+    // Listen for fullscreen changes
+    scene.scale.on('enterfullscreen', updateIcon);
+    scene.scale.on('leavefullscreen', updateIcon);
+
+    return { btn, icon };
+}
+
+export default { createFullscreenButton };