feat(web): add smooth page transitions and WASM loading states

- Implement navigation state tracking with optimistic UI updates
- Add loading spinner and error handling for WASM initialization
- Insert browser yield points during game initialization to prevent freezing
- Redesign leaderboard with tabbed navigation and mock data structure
- Add utility CSS classes for consistent page layouts

web/lib/navigation.ts

@@ -0,0 +1,24 @@
+import { useSyncExternalStore } from "react";
+
+type Listener = (pendingUrl: string | null) => void;
+
+let pendingUrl: string | null = null;
+const listeners = new Set<Listener>();
+
+export function setPendingNavigation(url: string | null) {
+  pendingUrl = url;
+  listeners.forEach((listener) => listener(pendingUrl));
+}
+
+export function getPendingNavigation(): string | null {
+  return pendingUrl;
+}
+
+export function subscribeToPendingNavigation(listener: Listener): () => void {
+  listeners.add(listener);
+  return () => listeners.delete(listener);
+}
+
+export function usePendingNavigation(): string | null {
+  return useSyncExternalStore(subscribeToPendingNavigation, getPendingNavigation, () => null);
+}