Improve reconnect grace period for rooms

Temporary connection loss should not immediately cause a user to lose their scramble or disrupt room state.

Feedback source:
- "It would be great if the server changed the timeout time, because if you lose the connection you instantly lose the scramble."

Acceptance ideas:
- Add a reconnect grace period before treating users as gone.
- Preserve current scramble/attempt state across short disconnects.
- Make reconnection behavior explicit for competitors and spectators.
- Ensure empty-room cleanup still works after the grace period.