feat(01-06): implement FeatureViewModelBase with async/cancel/progress pattern

- Add ProgressUpdatedMessage ValueChangedMessage for StatusBar live updates
- Add FeatureViewModelBase with CancellationTokenSource lifecycle, IsRunning,
  IProgress<OperationProgress>, OperationCanceledException handling
- Add 6 unit tests covering lifecycle, progress, cancellation, error handling
  and CanExecute guard

SharepointToolbox/Core/Messages/ProgressUpdatedMessage.cs

@@ -0,0 +1,9 @@
+using CommunityToolkit.Mvvm.Messaging.Messages;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Core.Messages;
+
+public sealed class ProgressUpdatedMessage : ValueChangedMessage<OperationProgress>
+{
+    public ProgressUpdatedMessage(OperationProgress progress) : base(progress) { }
+}