Handle board status message

This is another step to allow reconnecting after connection loss or
browser closing.

When the game is left with the X button on the bottom right, we will
close the websocket connection, to let the server know, that we are
gone.

The server still has issues that prevent this from working flawlessly.

@@ -1,4 +1,5 @@
 import 'package:go_router/go_router.dart';
+import 'package:mchess/connection/ws_connection.dart';