Stage 17 round 6 (#10 backend): enforce chat only on your turn

PostMessage now rejects a chat sent on a finished game or when it is not the sender's
turn (ErrChatNotYourTurn -> 409 chat_not_your_turn), matching the UI where the message
field is hidden off-turn and only the nudge shows. Existing chat tests post on the
to-move seat and are unaffected; adds an off-turn-rejection integration test + the dto
mapping case + the UI error message.

ui/src/lib/i18n/en.ts

@@ -158,6 +158,7 @@ export const en = {
   'error.no_hint_available': 'No options with your letters.',
   'error.chat_rejected': 'Message rejected (too long or contains contact info).',
   'error.nudge_too_soon': "Please don't rush your opponent so often.",
+  'error.chat_not_your_turn': 'You can chat only on your turn.',
   'error.game_finished': 'This game is finished.',
   'error.not_a_player': 'You are not a player in this game.',
   'error.already_queued': 'You are already in the queue.',