feat: add Dashboard as new startseite

Hero card with next match + countdown, stats tiles (rank, points, streak),
and contextual nudges. Replaces match list as landing page.

frontend/src/App.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect } from 'react';
 import { Routes, Route, NavLink } from 'react-router-dom';
 import { Sun, Moon, Settings } from 'lucide-react';
+import DashboardPage from './pages/DashboardPage';
 import MatchesPage from './pages/MatchesPage';
 import LeaderboardPage from './pages/LeaderboardPage';
 import ProfilePage from './pages/ProfilePage';
@@ -106,7 +107,7 @@ export default function App() {
 
       <main className={styles.main}>
         <Routes>
-          <Route path="/" element={<MatchesPage key={refreshKey} devUser={devUser} />} />
+          <Route path="/" element={<DashboardPage key={refreshKey} devUser={devUser} />} />
           <Route path="/spiele" element={<MatchesPage key={refreshKey} devUser={devUser} />} />
           <Route path="/rangliste" element={<LeaderboardPage key={refreshKey} />} />
           <Route path="/profil" element={<ProfilePage key={refreshKey} />} />