Announce the add-source validation message to screen readers

Author: GabrielDuf

src/UniGetUI.Avalonia/ViewModels/Pages/SettingsPages/SourceManagerCardViewModel.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Threading.Tasks;
+using Avalonia.Automation;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using UniGetUI.Avalonia.Infrastructure;
@@ -159,6 +160,9 @@ private void DeleteSource(IManagerSource source)
         Sources.Remove(source);
     }
 
+    partial void OnAddErrorChanged(string value)
+        => AccessibilityAnnouncementService.Announce(value, AutomationLiveSetting.Assertive);
+
     partial void OnNewSourceNameChanged(string value) => AddError = "";
 
     partial void OnNewSourceUrlChanged(string value) => AddError = "";