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/pages/_error/+Page.tsx

@@ -2,18 +2,10 @@ import { usePageContext } from "vike-react/usePageContext";
 
 export default function Page() {
   const { is404 } = usePageContext();
-  if (is404) {
-    return (
-      <>
-        <h1>Page Not Found</h1>
-        <p>This page could not be found.</p>
-      </>
-    );
-  }
   return (
-    <>
-      <h1>Internal Error</h1>
-      <p>Something went wrong.</p>
-    </>
+    <div className="flex flex-col items-center justify-center min-h-[50vh] text-center px-4">
+      <h1 className="text-4xl font-bold mb-4">{is404 ? "Page Not Found" : "Internal Error"}</h1>
+      <p className="text-gray-400">{is404 ? "This page could not be found." : "Something went wrong."}</p>
+    </div>
   );
 }