Route room inputs through one toRoom helper, iterate live seats with
connected(), and share seating and reconnect-window code. Declare every
error code once in errcodes.go, which the web vocabulary test now reads.
Fixes:
- a kicked player is released before being told, so their next action
is answered not_in_a_room
- a rejected word is counted before move_rejected is sent
- room code draw and registration share one lock hold
- two concurrent resumes with one token can no longer both take a seat
- multiplayer tests ready guests one at a time instead of racing
Behaviour changes:
- opening or joining another room while a game is running is refused
with already_in_a_game
- resigning in the lobby answers game_not_started
tiennm99
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