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

lib/dateUtils.ts

@@ -0,0 +1,8 @@
+export function getTodayISOString(timezone = process.env.TZ || 'Europe/Berlin'): string {
+    return new Date().toLocaleDateString('en-CA', {
+        timeZone: timezone,
+        year: 'numeric',
+        month: '2-digit',
+        day: '2-digit'
+    });
+}