style: Lucide Icons für Theme-Toggle + stärkerer Glossy-Effekt auf Flag-Boxen

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,5 +1,6 @@
 import { useState, useEffect } from 'react';
 import { Routes, Route, NavLink } from 'react-router-dom';
+import { Sun, Moon } from 'lucide-react';
 import MatchesPage from './pages/MatchesPage';
 import LeaderboardPage from './pages/LeaderboardPage';
 import ProfilePage from './pages/ProfilePage';
@@ -97,7 +98,7 @@ export default function App() {
               title={theme === 'dark' ? 'Light Mode aktivieren' : 'Dark Mode aktivieren'}
               aria-label="Theme wechseln"
             >
-              {theme === 'dark' ? '☀️' : '🌙'}
+              {theme === 'dark' ? <Sun size={16} /> : <Moon size={16} />}
             </button>
           </nav>
         </div>