🐛(frontend) avoid unnecessary refetches of room's data

LiveKit access tokens are valid for 6 hours after generation. Without a
`staleTime` set, room data becomes stale immediately (0 milliseconds),
causing unnecessary refetches, particularly when users switch focus back
to the call window.

This issue led to an excessive number of "get room" events, as observed
in the analytics. For example, during a meeting, if a user switches tabs
and then returns, the room data would be refetched despite the access
token still being valid.

By adding a `staleTime` aligned with the token's validity, we avoid
unnecessary network requests.

src/frontend/src/features/rooms/components/Conference.tsx

@@ -49,6 +49,7 @@ export const Conference = ({
     data,
   } = useQuery({
     queryKey: fetchKey,
+    staleTime: 6 * 60 * 60 * 1000, // By default, LiveKit access tokens expire 6 hours after generation
     enabled: !initialRoomData,
     initialData: initialRoomData,
     queryFn: () =>