fix: handle stale chunk 404s + add page loading states

- Router catches failed dynamic imports and forces reload (stale deploy cache)
- LeaderboardPage: sprite-based loading animation while fetching
- ArenaPage: sprite-based loading animation while fetching

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/router.ts

@@ -67,3 +67,11 @@ export const router = createRouter({
   history: createWebHistory(),
   routes,
 })
+
+// Handle stale chunk 404s after deploy — force reload to get new assets
+router.onError((err, to) => {
+  if (err.message.includes('Failed to fetch dynamically imported module') ||
+      err.message.includes('Importing a module script failed')) {
+    window.location.assign(to.fullPath)
+  }
+})