[client] Disconnect daemon on GUI quit via async Down

The tray Quit menu now disconnects the daemon before exiting instead of
only tearing down the GUI. A new DownAsync RPC lets the daemon start the
teardown and return immediately: beginDown cancels the connection under
the mutex (so it cannot reconnect), then finishDown (the retry-goroutine
wait and status reset) runs on a background goroutine. handleQuit aborts
any in-flight profile switch first (so a queued Up cannot reconnect during
teardown) and calls DownAsync so quitting never blocks on the engine
shutdown.

client/ui/services/connection.go

@@ -177,6 +177,17 @@ func (s *Connection) Down(ctx context.Context) error {
 	return nil
 }
 
+func (s *Connection) DownAsync(ctx context.Context) error {
+	cli, err := s.conn.Client()
+	if err != nil {
+		return err
+	}
+	if _, err = cli.DownAsync(ctx, &proto.DownAsyncRequest{}); err != nil {
+		return s.classifyDaemonError(err)
+	}
+	return nil
+}
+
 // OpenURL opens url in an external browser; the embedded webview blocks
 // window.open, so the SSO verification page can't pop inline. Honors $BROWSER
 // before the platform default.

client/ui/tray.go

@@ -30,6 +30,8 @@ const (
 
 	statusError = "Error"
 
+	quitDownTimeout = 5 * time.Second
+
 	urlGitHubRepo     = "https://github.com/netbirdio/netbird"
 	urlGitHubReleases = "https://github.com/netbirdio/netbird/releases/latest"
 	urlDocs           = "https://docs.netbird.io"
@@ -446,11 +448,28 @@ func (t *Tray) buildMenu() *application.Menu {
 	menu.AddSeparator()
 	menu.Add(t.loc.T("tray.menu.quit")).
 		SetAccelerator("CmdOrCtrl+Q").
-		OnClick(func(*application.Context) { t.app.Quit() })
+		OnClick(func(*application.Context) { t.handleQuit() })
 
 	return menu
 }
 
+func (t *Tray) handleQuit() {
+	t.profileMu.Lock()
+	if t.switchCancel != nil {
+		t.switchCancel()
+		t.switchCancel = nil
+	}
+	t.profileMu.Unlock()
+	t.svc.DaemonFeed.CancelProfileSwitch()
+
+	ctx, cancel := context.WithTimeout(context.Background(), quitDownTimeout)
+	defer cancel()
+	if err := t.svc.Connection.DownAsync(ctx); err != nil {
+		log.Errorf("disconnect on quit: %v", err)
+	}
+	t.app.Quit()
+}
+
 // handleConnect receives the clicked item from the buildMenu closure —
 // t.upItem is menuMu-guarded and must not be read here.
 func (t *Tray) handleConnect(upItem *application.MenuItem) {