feat(lobby): keep lobby/game caches fresh from any screen + invitation delta channel

Builds on the cross-screen cache work: the global stream handler now keeps both
caches current no matter which screen is mounted, and invitations become a live
delta channel so the lobby's invitations list is fresh from any screen too.

Client (boundary already started):
- advanceCached now also folds opponent_joined into a not-currently-viewed game's
  cache via a new pure reducer applyOpponentJoined (extracted and reused by the
  mounted game board), so opening an open game that filled while you were elsewhere
  is flash-free.
- patchLobbyInvitation upserts a still-pending invitation and removes a terminal
  one (started/declined/cancelled/expired); the global notify handler calls it on
  the invitation / invitation_update sub-kinds.

Invitations delta channel (no wire/gateway/connector change — the notification
already carries the full invitation with id/status/game_id end to end):
- notify: a new in-app-only NotifyInvitationUpdate sub + NotificationInvitationUpdate
  constructor (shares encoding with NotificationInvitation). The Telegram connector
  renders no message for it, so a decline/cancel never becomes an out-of-app push.
- lobby: emit the changed invitation to every participant on respond (accept/decline),
  on the final accept's game start, and on cancel — so each participant's lobby patches
  its list in place. The authoritative list holds only pending invitations, so the
  client's pending-vs-terminal rule matches it exactly.

Tests: applyOpponentJoined + patchLobbyInvitation unit tests (TDD), the
NotificationInvitationUpdate encoding unit test, and integration assertions that
decline/cancel/accept publish invitation_update to every participant. Full local
suite green (backend unit+integration, UI check/unit/build/bundle/e2e). Docs:
ARCHITECTURE §10 (notify catalog + in-app-only note) and UI_DESIGN updated.

backend/internal/notify/notify_test.go

@@ -181,6 +181,23 @@ func TestNotificationInvitationCarriesInvitation(t *testing.T) {
 	}
 }
 
+func TestNotificationInvitationUpdateCarriesTerminalInvitation(t *testing.T) {
+	uid := uuid.New()
+	inv := notify.InvitationSummary{ID: "inv-9", Variant: "scrabble_en", Status: "declined"}
+	in := notify.NotificationInvitationUpdate(uid, inv)
+	if in.Kind != notify.KindNotification {
+		t.Fatalf("kind = %q", in.Kind)
+	}
+	ev := fb.GetRootAsNotificationEvent(in.Payload, 0)
+	if string(ev.Kind()) != notify.NotifyInvitationUpdate {
+		t.Fatalf("sub-kind = %q, want %q", ev.Kind(), notify.NotifyInvitationUpdate)
+	}
+	got := ev.Invitation(nil)
+	if got == nil || string(got.Id()) != "inv-9" || string(got.Status()) != "declined" {
+		t.Fatalf("invitation wrong: %+v", got)
+	}
+}
+
 func TestNotificationAccountCarriesAccount(t *testing.T) {
 	uid := uuid.New()
 	in := notify.NotificationAccount(uid, notify.NotifyFriendRequest, notify.AccountRef{AccountID: "a-1", DisplayName: "Ann"})