Implement dynamic lobby polling with faster refresh rate and prevent concurrent requests

Added faster polling interval (700ms) for pre-game lobby to show joining players in near real-time, while keeping slower 2.5s refresh for active games. Implemented pending request flag to prevent concurrent API calls from overlapping. Added `cache: "no-store"` to fetch configuration to ensure fresh data on every request. Updated useEffect dependency array to re-initialize polling when game start state changes.

frontend/src/App.jsx

@@ -225,18 +225,23 @@ export default function App() {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [gameId]);
 
-  // ✅ Live refresh (Members/Meta) – damit neue Joiner ohne Reload sichtbar sind
-  // Für 5–6 Spieler reicht 2.5s völlig, ist "live genug" und schont Backend.
+  // ✅ Live refresh (Members/Meta) – Lobby schnell, laufendes Spiel genügsamer.
   useEffect(() => {
     if (!me || !gameId) return;
 
     let alive = true;
+    let pending = false;
+    const refreshInterval = gameMeta?.started ? 2500 : 700;
 
     const tick = async () => {
+      if (pending) return;
+      pending = true;
       try {
         await loadGameMeta(); // refresh members + winner meta
       } catch {
         // ignore
+      } finally {
+        pending = false;
       }
     };
 
@@ -246,14 +251,14 @@ export default function App() {
     const id = setInterval(() => {
       if (!alive) return;
       tick();
-    }, 2500);
+    }, refreshInterval);
 
     return () => {
       alive = false;
       clearInterval(id);
     };
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [me?.id, gameId]);
+  }, [me?.id, gameId, gameMeta?.started]);
 
   useEffect(() => {
     // wid kann auch "" sein (kein Sieger)