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/ViewModels/FeatureViewModelBase.cs

@@ -0,0 +1,82 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using CommunityToolkit.Mvvm.Messaging;
+using Microsoft.Extensions.Logging;
+using SharepointToolbox.Core.Messages;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Localization;
+
+namespace SharepointToolbox.ViewModels;
+
+public abstract partial class FeatureViewModelBase : ObservableRecipient
+{
+    private CancellationTokenSource? _cts;
+    private readonly ILogger<FeatureViewModelBase> _logger;
+
+    [ObservableProperty]
+    [NotifyCanExecuteChangedFor(nameof(CancelCommand))]
+    private bool _isRunning;
+
+    [ObservableProperty]
+    private string _statusMessage = string.Empty;
+
+    [ObservableProperty]
+    private int _progressValue;
+
+    public IAsyncRelayCommand RunCommand { get; }
+    public RelayCommand CancelCommand { get; }
+
+    protected FeatureViewModelBase(ILogger<FeatureViewModelBase> logger)
+    {
+        _logger = logger;
+        RunCommand = new AsyncRelayCommand(ExecuteAsync, () => !IsRunning);
+        CancelCommand = new RelayCommand(() => _cts?.Cancel(), () => IsRunning);
+        IsActive = true; // Activates ObservableRecipient for WeakReferenceMessenger
+    }
+
+    private async Task ExecuteAsync()
+    {
+        _cts = new CancellationTokenSource();
+        IsRunning = true;
+        StatusMessage = string.Empty;
+        ProgressValue = 0;
+        try
+        {
+            var progress = new Progress<OperationProgress>(p =>
+            {
+                ProgressValue = p.Total > 0 ? (int)(100.0 * p.Current / p.Total) : 0;
+                StatusMessage = p.Message;
+                WeakReferenceMessenger.Default.Send(new ProgressUpdatedMessage(p));
+            });
+            await RunOperationAsync(_cts.Token, progress);
+        }
+        catch (OperationCanceledException)
+        {
+            StatusMessage = TranslationSource.Instance["status.cancelled"];
+            _logger.LogInformation("Operation cancelled by user.");
+        }
+        catch (Exception ex)
+        {
+            StatusMessage = $"{TranslationSource.Instance["err.generic"]} {ex.Message}";
+            _logger.LogError(ex, "Operation failed.");
+        }
+        finally
+        {
+            IsRunning = false;
+            _cts?.Dispose();
+            _cts = null;
+        }
+    }
+
+    protected abstract Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress);
+
+    protected override void OnActivated()
+    {
+        Messenger.Register<TenantSwitchedMessage>(this, (r, m) => ((FeatureViewModelBase)r).OnTenantSwitched(m.Value));
+    }
+
+    protected virtual void OnTenantSwitched(TenantProfile profile)
+    {
+        // Derived classes override to reset their state
+    }
+}