feat(04-08,04-09): create Transfer/BulkMembers/BulkSites/FolderStructure ViewModels and Views

- TransferViewModel: source/dest site selection, Copy/Move mode, conflict policy, export failed
- TransferView: SitePickerDialog and FolderBrowserDialog wiring, confirm dialog
- BulkMembersViewModel, BulkSitesViewModel: CSV import, validate, preview, execute, retry, export
- FolderStructureViewModel: CSV import, site URL + library inputs, folder creation
- All 3 bulk Views: ConfirmBulkOperationDialog wiring, DataGrid preview with validation status
- Added EnumBoolConverter, StringToVisibilityConverter, ListToStringConverter to converters file

SharepointToolbox/Views/Converters/IndentConverter.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Globalization;
 using System.Windows;
 using System.Windows.Data;
@@ -46,3 +47,52 @@ public class InverseBoolConverter : IValueConverter
         => value is bool b && !b;
 }
 
+/// <summary>
+/// Converts a string to Visibility: Visible when non-empty, Collapsed when null or empty.
+/// </summary>
+[ValueConversion(typeof(string), typeof(Visibility))]
+public class StringToVisibilityConverter : IValueConverter
+{
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        => value is string s && !string.IsNullOrEmpty(s) ? Visibility.Visible : Visibility.Collapsed;
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        => throw new NotImplementedException();
+}
+
+/// <summary>
+/// Converts an enum value to bool by comparing it with the ConverterParameter.
+/// Used for RadioButton IsChecked bindings to enum properties.
+/// </summary>
+public class EnumBoolConverter : IValueConverter
+{
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+    {
+        if (value == null || parameter == null) return false;
+        return value.ToString() == parameter.ToString();
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+    {
+        if (value is bool b && b && parameter != null)
+            return Enum.Parse(targetType, parameter.ToString()!);
+        return System.Windows.DependencyProperty.UnsetValue;
+    }
+}
+
+/// <summary>
+/// Converts a List&lt;string&gt; to a single semicolon-separated string for display in DataGrid cells.
+/// </summary>
+public class ListToStringConverter : IValueConverter
+{
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+    {
+        if (value is IEnumerable<string> list)
+            return string.Join("; ", list);
+        return string.Empty;
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        => throw new NotImplementedException();
+}
+