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.

ui/src/lib/mock/client.ts

@@ -324,6 +324,15 @@ export class MockGateway implements GatewayClient {
   }
   async complaint(): Promise<void> {}
 
+  // Hide a finished game from the caller's list (Stage 17): drop it from the in-memory store so a
+  // subsequent gamesList omits it, mirroring the backend's per-account, finished-only rule.
+  async hideGame(gameId: string): Promise<void> {
+    const g = this.game(gameId);
+    if (g.view.status !== 'finished') throw new GatewayError('game_active');
+    this.games.delete(gameId);
+    this.drafts.delete(gameId);
+  }
+
   // --- draft (Stage 17): an in-memory composition store, so the reload/off-turn flow is
   // exercised without a backend. A committed move clears the actor's own draft, as on the server.
   async draftGet(gameId: string): Promise<string> {