🐛(frontend) fall back to user.full_name on request-entry

Since the username refactoring, the username in the store could be
undefined when the join input was pre-filled from user.full_name,
because no keystroke was needed to populate the store.

This led to a 400 error on the request-entry endpoint whenever the
user joined without editing the pre-filled name.

Fall back to user.full_name when the store username is missing, so
the endpoint always receives a value.

Acknowledged as a somewhat wobbly fix, but ships as-is until the
underlying flow is reworked.

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

@@ -316,7 +316,7 @@ export const Join = ({
     refetch: refetchRoom,
   } = useQuery({
     queryKey: [keys.room, roomId],
-    queryFn: () => fetchRoom({ roomId, username }),
+    queryFn: () => fetchRoom({ roomId, username: username || user?.full_name }),
     staleTime: 6 * 60 * 60 * 1000, // By default, LiveKit access tokens expire 6 hours after generation
     retry: false,
     enabled: false,
@@ -339,7 +339,7 @@ export const Join = ({
 
   const { status, startWaiting } = useLobby({
     roomId,
-    username,
+    username: username || user?.full_name,
     onAccepted: handleAccepted,
   })