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/transcode/transcode.go

@@ -41,6 +41,7 @@ const (
 	MsgChatNudge      = "chat.nudge"
 	MsgDraftGet       = "draft.get"
 	MsgDraftSave      = "draft.save"
+	MsgGameHide       = "game.hide"
 )
 
 // Request is one decoded Execute call.
@@ -113,6 +114,7 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLan
 	r.ops[MsgChatNudge] = Op{Handler: nudgeHandler(backend), Auth: true}
 	r.ops[MsgDraftGet] = Op{Handler: getDraftHandler(backend), Auth: true}
 	r.ops[MsgDraftSave] = Op{Handler: saveDraftHandler(backend), Auth: true}
+	r.ops[MsgGameHide] = Op{Handler: hideGameHandler(backend), Auth: true}
 	registerStage8(r, backend)
 	registerStage11(r, backend, tg, defaultLanguages)
 	return r
@@ -451,3 +453,15 @@ func saveDraftHandler(backend *backendclient.Client) Handler {
 		return encodeDraftView(""), nil
 	}
 }
+
+// hideGameHandler hides a finished game from the caller's own list (Stage 17). It reuses
+// GameActionRequest for the game id and echoes an Ack.
+func hideGameHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsGameActionRequest(req.Payload, 0)
+		if err := backend.HideGame(ctx, req.UserID, string(in.GameId())); err != nil {
+			return nil, err
+		}
+		return encodeAck(true), nil
+	}
+}