feat(04-09): create BulkMembers, BulkSites, and FolderStructure ViewModels and Views

- BulkMembersViewModel: CSV import, validate, preview, confirm, execute, retry failed, export failed
- BulkSitesViewModel: same flow using IBulkSiteService.CreateSitesAsync
- FolderStructureViewModel: site URL + library inputs, CSV folders, FolderStructureService.BuildUniquePaths
- BulkMembersView/BulkSitesView/FolderStructureView: XAML + code-behind wiring ConfirmBulkOperationDialog
- [Rule 3] Fixed duplicate converter definitions: removed untracked standalone EnumBoolConverter/StringToVisibilityConverter/ListToStringConverter files (already defined in IndentConverter.cs)

SharepointToolbox/Views/Tabs/BulkMembersView.xaml.cs

@@ -0,0 +1,21 @@
+using System.Windows.Controls;
+using SharepointToolbox.Views.Dialogs;
+
+namespace SharepointToolbox.Views.Tabs;
+
+public partial class BulkMembersView : UserControl
+{
+    public BulkMembersView(ViewModels.Tabs.BulkMembersViewModel viewModel)
+    {
+        InitializeComponent();
+        DataContext = viewModel;
+
+        viewModel.ShowConfirmDialog = message =>
+        {
+            var dlg = new ConfirmBulkOperationDialog(message)
+                { Owner = System.Windows.Window.GetWindow(this) };
+            dlg.ShowDialog();
+            return dlg.IsConfirmed;
+        };
+    }
+}