Add floating version badge in top-left corner

Small yellow badge fixed at position (4, 4) showing the running build
version. Helps visually confirm which bundle a browser is loading when
diagnosing cache issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/map/MapLayout.tsx

@@ -71,6 +71,7 @@ export const MapLayout: React.FC<Props> = ({ data }) => {
           socket={data.socketRef.current} />
         <RareNotification recentRares={data.recentRares} />
         <DeathNotification deathAlerts={data.deathAlerts} />
+        {version && <div className="ml-version-badge">v{version}</div>}
       </div>
     </WindowManagerProvider>
   );