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/api/daily/route.ts

@@ -1,18 +1,12 @@
 import { NextResponse } from 'next/server';
 import { PrismaClient } from '@prisma/client';
+import { getTodayISOString } from '@/lib/dateUtils';
 
 const prisma = new PrismaClient();
 
 export async function GET() {
     try {
-        // Use timezone from environment variable (default: Europe/Berlin)
-        const timezone = process.env.TZ || 'Europe/Berlin';
-        const today = new Date().toLocaleDateString('en-CA', {
-            timeZone: timezone,
-            year: 'numeric',
-            month: '2-digit',
-            day: '2-digit'
-        }); // Format: "2025-11-21"
+        const today = getTodayISOString();
 
         let dailyPuzzle = await prisma.dailyPuzzle.findUnique({
             where: { date: today },