fix(chess): optimistic moves and forfeit UI feedback

- Add localFen/localFenRef in ChessBoard for optimistic piece placement,
  preventing snap-back while awaiting server confirmation
- Sync localFen from server state on each chess.fen update
- Guard GAME_STATE handler in useGameRoom from overwriting a finished
  roomStatus, fixing the race where GAME_ENDED (pub/sub) arrives before
  GAME_STATE (direct ws.send)
- Reset confirmForfeit immediately on forfeit dispatch for instant UI feedback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

panel/src/lib/useGameRoom.ts

@@ -58,7 +58,11 @@ export function useGameRoom(roomId: string, userId: string, role?: string, prefe
                     break;
 
                 case "GAME_STATE":
-                    setState(prev => ({ ...prev, gameState: msg.state, roomStatus: "playing" }));
+                    setState(prev => ({
+                        ...prev,
+                        gameState: msg.state,
+                        roomStatus: prev.roomStatus === "finished" ? "finished" : "playing",
+                    }));
                     break;
 
                 case "GAME_STARTED":