Add backend health check, fetch timeouts, stale token cleanup, and error screen

Frontend now checks /health before starting auth flow. Shows a clear
"Cannot Connect to Server" screen with retry button when backend is
unreachable. Stale non-JWT tokens in localStorage are detected and
cleared automatically. All API calls have a 10s timeout via AbortController.

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

apps/web/src/hooks/useCloudSave.ts

@@ -1,6 +1,6 @@
 import { useEffect, useRef } from 'react';
 import { useGameStore } from '@/store';
-import { api, getAuthToken, setAuthToken } from '@/lib/api';
+import { api, getAuthToken, setAuthToken, clearAuthToken, decodeTokenPayload } from '@/lib/api';
 import { AUTO_SAVE_INTERVAL_TICKS } from '@ai-tycoon/shared';
 
 export function useCloudSave() {
@@ -31,8 +31,11 @@ export function useCloudSave() {
 }
 
 export async function ensureAuth(): Promise<string | null> {
-  let token = getAuthToken();
-  if (token) return token;
+  const token = getAuthToken();
+  if (token) {
+    if (decodeTokenPayload(token)) return token;
+    clearAuthToken();
+  }
 
   try {
     const result = await api.auth.anonymous();