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/store.go

@@ -186,6 +186,23 @@ func (s *Store) ListGamesForAccount(ctx context.Context, accountID uuid.UUID) ([
 	if len(grows) == 0 {
 		return nil, nil
 	}
+	// Drop games the account has hidden from its own lobby (Stage 17).
+	hidden, err := s.hiddenGameIDs(ctx, accountID)
+	if err != nil {
+		return nil, err
+	}
+	if len(hidden) > 0 {
+		kept := grows[:0]
+		for _, g := range grows {
+			if !hidden[g.GameID] {
+				kept = append(kept, g)
+			}
+		}
+		grows = kept
+		if len(grows) == 0 {
+			return nil, nil
+		}
+	}
 
 	ids := make([]postgres.Expression, len(grows))
 	for i, g := range grows {
@@ -215,6 +232,36 @@ func (s *Store) ListGamesForAccount(ctx context.Context, accountID uuid.UUID) ([
 	return out, nil
 }
 
+// HideGame hides a game from the account's own lobby list (idempotent). The caller validates the
+// game is finished and the account is a player (Stage 17).
+func (s *Store) HideGame(ctx context.Context, accountID, gameID uuid.UUID) error {
+	_, err := s.db.ExecContext(ctx,
+		`INSERT INTO backend.game_hidden (account_id, game_id) VALUES ($1, $2) ON CONFLICT DO NOTHING`,
+		accountID, gameID)
+	if err != nil {
+		return fmt.Errorf("game: hide game: %w", err)
+	}
+	return nil
+}
+
+// hiddenGameIDs returns the set of games the account has hidden from its lobby.
+func (s *Store) hiddenGameIDs(ctx context.Context, accountID uuid.UUID) (map[uuid.UUID]bool, error) {
+	rows, err := s.db.QueryContext(ctx, `SELECT game_id FROM backend.game_hidden WHERE account_id = $1`, accountID)
+	if err != nil {
+		return nil, fmt.Errorf("game: hidden ids: %w", err)
+	}
+	defer rows.Close()
+	out := map[uuid.UUID]bool{}
+	for rows.Next() {
+		var id uuid.UUID
+		if err := rows.Scan(&id); err != nil {
+			return nil, fmt.Errorf("game: scan hidden id: %w", err)
+		}
+		out[id] = true
+	}
+	return out, rows.Err()
+}
+
 // ListGames returns games for the admin games list, most-recently-updated first,
 // paginated. status filters by lifecycle ("active"/"finished") when non-empty.
 // The seats are not loaded — the list shows summaries; the detail view uses