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.

backend/internal/game/service.go

@@ -727,6 +727,30 @@ func (svc *Service) ListForAccount(ctx context.Context, accountID uuid.UUID) ([]
 	return svc.store.ListGamesForAccount(ctx, accountID)
 }
 
+// HideGame hides a finished game from accountID's own lobby (it stays visible to the other
+// players); it is irreversible by design. Only a player of a finished game may hide it
+// (ErrNotAPlayer / ErrGameActive otherwise); hiding an already-hidden game is a no-op (Stage 17).
+func (svc *Service) HideGame(ctx context.Context, accountID, gameID uuid.UUID) error {
+	g, err := svc.store.GetGame(ctx, gameID)
+	if err != nil {
+		return err
+	}
+	seated := false
+	for _, s := range g.Seats {
+		if s.AccountID == accountID {
+			seated = true
+			break
+		}
+	}
+	if !seated {
+		return ErrNotAPlayer
+	}
+	if g.Status != StatusFinished {
+		return ErrGameActive
+	}
+	return svc.store.HideGame(ctx, accountID, gameID)
+}
+
 // GameByID returns a game with its seats for the admin console detail view.
 func (svc *Service) GameByID(ctx context.Context, id uuid.UUID) (Game, error) {
 	return svc.store.GetGame(ctx, id)