front-end: fix repeated disconnects on close

front-end: fix repeated disconnects on close

When the front-end connects to the back-end, it creates a callback that
sends a disconnect message with the received ID on "pagehide" and
"beforeunload" events. The previous implementation did not "undo" these
callbacks when the game was reset and a new ID was received. This fix
prevents the front-end from resending disconnect messages with
previously received IDs on browser window/tab close.