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/App.xaml

@@ -3,6 +3,6 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:local="clr-namespace:SharepointToolbox">
     <Application.Resources>
-
+        <BooleanToVisibilityConverter x:Key="BoolToVisibilityConverter" />
     </Application.Resources>
 </Application>