feat(01-06): build WPF shell — MainWindow XAML, ViewModels, LogPanelSink wiring

- Add FeatureTabBase UserControl with ProgressBar/TextBlock/CancelButton strip
  (Visibility bound to IsRunning, shown only during operations)
- Add MainWindowViewModel with TenantProfiles ObservableCollection, ConnectCommand,
  ClearSessionCommand, ManageProfilesCommand, ProgressUpdatedMessage subscription
- Add ProfileManagementViewModel wrapping ProfileService CRUD with input validation
- Add SettingsViewModel (extends FeatureViewModelBase) with language/folder settings
- Update MainWindow.xaml: DockPanel shell with Toolbar, TabControl (8 tabs), 150px
  RichTextBox LogPanel, StatusBar (tenant name | ProgressStatus | ProgressPercentage)
- MainWindow.xaml.cs: DI constructor, DataContext=viewModel, LoadProfilesAsync on Loaded
- App.xaml.cs: register all services, wire LogPanelSink after MainWindow resolved,
  register DispatcherUnhandledException and UnobservedTaskException global handlers
- App.xaml: add BoolToVisibilityConverter resource

SharepointToolbox/MainWindow.xaml.cs

@@ -1,23 +1,26 @@
-using System.Text;
 using System.Windows;
 using System.Windows.Controls;
-using System.Windows.Data;
-using System.Windows.Documents;
-using System.Windows.Input;
-using System.Windows.Media;
-using System.Windows.Media.Imaging;
-using System.Windows.Navigation;
-using System.Windows.Shapes;
+using SharepointToolbox.ViewModels;
 
 namespace SharepointToolbox;
 
-/// <summary>
-/// Interaction logic for MainWindow.xaml
-/// </summary>
 public partial class MainWindow : Window
 {
-    public MainWindow()
+    private readonly MainWindowViewModel _viewModel;
+
+    public MainWindow(MainWindowViewModel viewModel)
     {
         InitializeComponent();
+        _viewModel = viewModel;
+        DataContext = viewModel;
+        Loaded += OnLoaded;
     }
-}
+
+    private async void OnLoaded(object sender, RoutedEventArgs e)
+    {
+        await _viewModel.LoadProfilesAsync();
+    }
+
+    // Expose the LogPanel RichTextBox (generated by x:Name="LogPanel") for LogPanelSink wiring
+    public RichTextBox GetLogPanel() => LogPanel;
+}