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/gameState.ts

@@ -1,6 +1,7 @@
 'use client';
 
 import { useState, useEffect } from 'react';
+import { getTodayISOString } from './dateUtils';
 
 export interface GameState {
     date: string;
@@ -31,7 +32,7 @@ export function useGameState() {
     useEffect(() => {
         // Load game state
         const stored = localStorage.getItem(STORAGE_KEY);
-        const today = new Date().toISOString().split('T')[0];
+        const today = getTodayISOString();
 
         if (stored) {
             const parsed: GameState = JSON.parse(stored);