FEATURE: Migrate username storage from localStorage to IndexedDB

- Created storage.js wrapper with IndexedDB support
- Automatic fallback to localStorage if IndexedDB unavailable
- Updated all username storage calls to use async API
- Better performance and more storage capacity
- Improved PWA compatibility

Benefits:
- 50MB+ storage vs 5-10MB localStorage
- Async API doesn't block main thread
- Better for future features (game history, stats, etc.)

index.html

@@ -182,6 +182,7 @@
       </div>
     </div>
 
+    <script src="storage.js"></script>
     <script src="multiplayer.js"></script>
     <script src="game.js"></script>
     <script>