fix(client-ui): resolve Medium code-review finding (Client.UI-001)

Route the synchronous IsLoading = true write through _dispatcher.Post so
both IsLoading assignments use the same dispatch path as Results.Clear()
and the final IsLoading = false, eliminating the ordering hazard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/ViewModels/HistoryViewModel.cs

@@ -73,7 +73,7 @@ public partial class HistoryViewModel : ObservableObject
     {
         if (string.IsNullOrEmpty(SelectedNodeId)) return;
 
-        IsLoading = true;
+        _dispatcher.Post(() => IsLoading = true);
         _dispatcher.Post(() => Results.Clear());
 
         try