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

app/page.tsx

@@ -1,4 +1,5 @@
 import Game from '@/components/Game';
+import { getTodayISOString } from '@/lib/dateUtils';
 
 export const dynamic = 'force-dynamic';
 
@@ -14,7 +15,7 @@ if (process.env.NODE_ENV !== 'production') globalForPrisma.prisma = prisma;
 
 async function getDailyPuzzle() {
   try {
-    const today = new Date().toISOString().split('T')[0];
+    const today = getTodayISOString();
     console.log(`[getDailyPuzzle] Checking puzzle for date: ${today}`);
 
     let dailyPuzzle = await prisma.dailyPuzzle.findUnique({