fix: sync public channel preview updates

internal/app/dialogs/service.go

@@ -342,11 +342,24 @@ func (s *Service) appendMissingChannelPeerPreviews(ctx context.Context, userID i
 		if !ok || view.Forbidden {
 			continue
 		}
+		if view.Self.Status == domain.ChannelMemberLeft && !view.Self.Guest {
+			// A visible public preview still needs one transient dialog so a
+			// client can finish bootstrapping the requested peer. Keep the top
+			// message and read state at zero: the response is an admission
+			// token, not a persisted/chat-list dialog snapshot. In particular,
+			// clients that persist non-zero top dialogs will instead continue
+			// with messages.getHistory, which is the authoritative preview
+			// history path.
+			out.Dialogs = append(out.Dialogs, publicChannelPreviewBootstrapDialog(view))
+			out.Channels = append(out.Channels, view.Channel)
+			out.Count++
+			present[channelID] = struct{}{}
+			continue
+		}
 		// Linked discussion guests need a transient peer-dialog snapshot so
 		// TDesktop can finish materializing the comments History after
 		// requestSelf. ChannelLeft keeps the snapshot out of the main chat list,
-		// and Guest guarantees this path never turns an ordinary public preview
-		// into a dialog.
+		// while Guest authorizes the target's real top-message snapshot.
 		if view.Self.Status != domain.ChannelMemberActive && !view.Self.Guest {
 			continue
 		}
@@ -378,6 +391,14 @@ func (s *Service) appendMissingChannelPeerPreviews(ctx context.Context, userID i
 	return out, nil
 }
 
+func publicChannelPreviewBootstrapDialog(view domain.ChannelView) domain.Dialog {
+	return domain.Dialog{
+		Peer:        domain.Peer{Type: domain.PeerTypeChannel, ID: view.Channel.ID},
+		ChannelLeft: true,
+		Pts:         view.Channel.Pts,
+	}
+}
+
 func isChannelPreviewAccessError(err error) bool {
 	return errors.Is(err, domain.ErrChannelPrivate) ||
 		errors.Is(err, domain.ErrChannelUserBanned) ||