feat(03-08): create DuplicatesViewModel, DuplicatesView XAML and code-behind

- DuplicatesViewModel: ModeFiles/Folders, criteria checkboxes, group flattening to DuplicateRow
- Uses TenantProfile site URL override pattern (ctx.Url is read-only)
- ExportHtmlCommand exports DuplicateGroup list via DuplicatesHtmlExportService
- DuplicatesView.xaml: type selector, criteria panel + flattened DataGrid
- DuplicatesView.xaml.cs: DI constructor with DataContext wiring

SharepointToolbox/Views/Tabs/DuplicatesView.xaml.cs

@@ -0,0 +1,12 @@
+using System.Windows.Controls;
+
+namespace SharepointToolbox.Views.Tabs;
+
+public partial class DuplicatesView : UserControl
+{
+    public DuplicatesView(ViewModels.Tabs.DuplicatesViewModel viewModel)
+    {
+        InitializeComponent();
+        DataContext = viewModel;
+    }
+}