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_test.go

@@ -150,6 +150,39 @@ func gameActionPayload(gameID string) []byte {
 	return b.FinishedBytes()
 }
 
+// TestHideGameForwardsToBackend checks game.hide reuses GameActionRequest, POSTs to the
+// game's /hide endpoint with the caller's id, and echoes an Ack (Stage 17).
+func TestHideGameForwardsToBackend(t *testing.T) {
+	var hit bool
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		hit = true
+		if r.Method != http.MethodPost || r.URL.Path != "/api/v1/user/games/g-1/hide" {
+			t.Errorf("unexpected %s %q", r.Method, r.URL.Path)
+		}
+		if got := r.Header.Get("X-User-ID"); got != "u-1" {
+			t.Errorf("X-User-ID = %q, want u-1", got)
+		}
+		_, _ = w.Write([]byte(`{"ok":true}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, ok := reg.Lookup(transcode.MsgGameHide)
+	if !ok {
+		t.Fatal("game.hide not registered")
+	}
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1", Payload: gameActionPayload("g-1")})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	if !hit {
+		t.Error("backend not called")
+	}
+	if ack := fb.GetRootAsAck(payload, 0); !ack.Ok() {
+		t.Error("ack not ok")
+	}
+}
+
 func TestGamesListRoundTripDecodesSeatNames(t *testing.T) {
 	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
 		if got := r.Header.Get("X-User-ID"); got != "u-9" {