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.

gateway/internal/backendclient/api.go

@@ -354,6 +354,12 @@ func (c *Client) SaveDraft(ctx context.Context, userID, gameID string, body json
 	return c.do(ctx, http.MethodPut, c.gamePath(gameID, "/draft"), userID, "", body, nil)
 }
 
+// HideGame hides a finished game from the caller's own games list (Stage 17). The action is
+// per-account and irreversible; the game stays visible to the other players.
+func (c *Client) HideGame(ctx context.Context, userID, gameID string) error {
+	return c.do(ctx, http.MethodPost, c.gamePath(gameID, "/hide"), userID, "", struct{}{}, nil)
+}
+
 // Evaluate previews a tentative play's legality and score. The tiles are addressed by
 // alphabet index (Stage 13).
 func (c *Client) Evaluate(ctx context.Context, userID, gameID, dir string, tiles []PlayTileJSON) (EvalResultResp, error) {