feat(chat): limit in-game chat to one message per turn

Enforce one chat message per turn on both ends. The backend rejects a
second message in the same turn (ErrChatAlreadySentThisTurn -> 409
chat_already_sent), keyed on the move-driven turn start (turn_started_at).
The UI derives "already wrote this turn" from the message list against
GameView.lastActivityUnix (no counter, survives reopening, resets on turn
change), hides the field behind a short caption once the limit is reached,
and now reloads the game state on turn/game-state events so the toggle and
the limit follow the live game. Enter is gated on !busy to avoid a
double-send in the in-flight window.

Backend: new game.TurnStartedAt; social GameReader gains it; PostMessage
enforces the limit reusing lastMessageAt. UI: new lib/chatlimit.ts pure
logic + unit tests; Chat/ChatScreen wiring; chat.sentThisTurn and
error.chat_already_sent i18n (en/ru); extended chat e2e. Docs: FUNCTIONAL
(+ru), ARCHITECTURE, UI_DESIGN.

docs/FUNCTIONAL.md

@@ -155,7 +155,9 @@ and/or friend requests — and block individual players (a per-user block hides
 person's chat and stops requests and game invitations both ways; it also ends any
 existing friendship). Per-game chat is for quick reactions: messages are short
 (up to 60 characters) and may not contain links, email addresses or phone numbers,
-even disguised. Nudge the player whose turn is awaited at most once per hour (the
+even disguised. You may send **one message per turn, on your own turn**; once it is sent
+the field gives way to a short caption until your next turn. Nudge the player whose turn
+is awaited at most once per hour (the
 nudge is part of the game chat); the out-of-app push is delivered via the platform.
 Chat and the word-check tool share one **comms screen** with **💬 chat** / **🔎 dictionary**
 tabs, reached from the 💬 in the move-history header (with a back to the game); a new chat