Fix daily puzzle rotation timezone issue

- Added lib/dateUtils.ts for consistent timezone handling
- Updated app/page.tsx and app/api/daily/route.ts to use Europe/Berlin timezone
- Updated lib/gameState.ts to sync client-side daily check with server time
- Exposed TZ env var to client in next.config.ts

next.config.ts

@@ -9,6 +9,9 @@ const nextConfig: NextConfig = {
       bodySizeLimit: '50mb',
     },
   },
+  env: {
+    TZ: process.env.TZ || 'Europe/Berlin',
+  },
   async headers() {
     return [
       {