feat(03-08): create SearchViewModel, SearchView XAML and code-behind

- SearchViewModel: full filter props, RunOperationAsync via ISearchService
- Uses TenantProfile site URL override pattern (ctx.Url is read-only)
- ExportCsvCommand + ExportHtmlCommand with CanExport guard
- SearchView.xaml: filter panel + DataGrid with all 8 columns
- SearchView.xaml.cs: DI constructor with DataContext wiring

SharepointToolbox/Views/Tabs/SearchView.xaml.cs

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