fix: add missing using Avalonia.Threading for DispatcherTimer

Author: J.A.R.V.I.S.

src/HaDeskLink/Views/NotificationPopup.cs

@@ -8,6 +8,7 @@
 using Avalonia.Controls;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Threading;
 using System;
 using System.Collections.Generic;
 using System.Linq;