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/server/handlers.go

@@ -68,6 +68,7 @@ func (s *Server) registerRoutes() {
 		u.GET("/games/:id/gcg", s.handleExportGCG)
 		u.GET("/games/:id/draft", s.handleGetDraft)
 		u.PUT("/games/:id/draft", s.handleSaveDraft)
+		u.POST("/games/:id/hide", s.handleHideGame)
 	}
 	if s.matchmaker != nil {
 		u.POST("/lobby/enqueue", s.handleEnqueue)