feat: desktop layout optimization

Dashboard: 2-column layout (hero left 60%, stats+nudges right 40%)
Spielplan: 2-column grid for match cards on desktop
Profile: wider max-width (900px), 6-column achievement grid
Header: Admin link back in desktop nav, max-width 1200px
All via CSS media queries (min-width: 768px), no HTML restructuring.

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

frontend/src/App.tsx

@@ -117,6 +117,9 @@ export default function App() {
             <NavLink to="/profil" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
               Mein Profil
             </NavLink>
+            <NavLink to="/admin" className={({ isActive }) => isActive ? styles.navLinkActive : styles.navLink}>
+              Admin
+            </NavLink>
           </nav>
           <div className={styles.headerActions}>
             <button