Add user stats feature with API and modal integration

Introduced an endpoint to fetch user stats and integrated it with a new StatsModal component in the frontend. Users can now view game statistics, including played games, wins, losses, and win rates, accessible from the user menu.

frontend/src/components/WinnerCard.jsx

@@ -25,7 +25,7 @@ export default function WinnerCard({
             <option value="">— kein Sieger —</option>
             {members.map((m) => (
               <option key={m.id} value={m.id}>
-                {m.email}
+                {m.display_name}
               </option>
             ))}
           </select>