Stage 17 #5: hide finished games from your own lobby list

A player can remove a finished game from their own 'my games' list. The action is
per-account, finished-only and irreversible (the game stays for the other players;
there is no un-hide).

- backend: migration 00012 game_hidden(account_id, game_id); store HideGame +
  hiddenGameIDs + ListGamesForAccount filtering; service HideGame (seat + finished
  checks, reusing ErrNotAPlayer / ErrGameActive); POST /api/v1/user/games/:id/hide.
- gateway: game.hide edge op (reuses GameActionRequest -> Ack) + backendclient.HideGame.
- ui: finished rows reveal a delete via swipe-left (touch) or a kebab tap (desktop),
  active rows get an inert chevron for icon alignment; optimistic removal + lobby-cache
  sync; mock + transport + client wiring; lobby.hideGame label (en/ru).
- tests: integration (active->ErrGameActive, outsider->ErrNotAPlayer, per-account,
  idempotent), gateway transcode round-trip, mock e2e (kebab -> delete); hardened a
  pre-existing chat-screen .back transition flake surfaced by the new test's timing.
- docs: ARCHITECTURE persistence list, FUNCTIONAL (+ _ru) lobby story, PLAN tracker.

docs/FUNCTIONAL.md

@@ -62,7 +62,10 @@ Bottom tab menu: **my games**, **profile**. The **my games** list groups games i
 sections — *your turn*, *opponent's turn* and *finished* (empty sections are hidden) — and
 orders them so the games awaiting your move come first, the longest-waiting on top, while
 opponent-turn and finished games are most-recent first; it renders as a compact,
-line-separated list (Stage 17). The game types offered on **New Game** are
+line-separated list (Stage 17). You can **remove a finished game from your own list**:
+swipe a finished row left (or, on desktop, tap its **⋮**) to reveal a **❌**, then tap it.
+The removal is per-account and permanent — the game disappears only from your list and stays
+in the other players' lists, and there is no undo. The game types offered on **New Game** are
 limited to the languages the player's sign-in service supports (English → Scrabble;
 Russian → Scrabble + Erudite; a bilingual service shows all three, and the web client is
 unrestricted). Variants are shown by their **display name** — both Scrabble variants read