feat(02-06): implement PermissionsViewModel with multi-site scan and SitePickerDialog

- PermissionsViewModel extends FeatureViewModelBase, implements RunOperationAsync
- Multi-site mode: loops SelectedSites; single-site mode: uses SiteUrl
- ExportCsvCommand and ExportHtmlCommand enabled only when Results.Count > 0
- OpenSitePickerCommand uses dialog factory pattern (Func<Window>?)
- OnTenantSwitched clears Results, SiteUrl, SelectedSites
- Flat ObservableProperty booleans (IncludeInherited, ScanFolders, etc.) build ScanOptions record
- SitePickerDialog XAML: filterable list with CheckBox column, Title, URL columns
- SitePickerDialog code-behind: loads sites on Window.Loaded, exposes SelectedUrls
- ISessionManager interface extracted for testability (SessionManager implements it)
- StartScanAsync_WithMultipleSiteUrls_CallsServiceOncePerUrl test passes (60/60 + 3 skip)

SharepointToolbox.Tests/ViewModels/PermissionsViewModelTests.cs

@@ -6,6 +6,7 @@ using Microsoft.SharePoint.Client;
 using Moq;
 using SharepointToolbox.Core.Models;
 using SharepointToolbox.Services;
+using SharepointToolbox.ViewModels;
 using SharepointToolbox.ViewModels.Tabs;
 
 namespace SharepointToolbox.Tests.ViewModels;
@@ -40,7 +41,7 @@ public class PermissionsViewModelTests
             mockPermissionsService.Object,
             mockSiteListService.Object,
             mockSessionManager.Object,
-            new NullLogger<PermissionsViewModel>());
+            new NullLogger<FeatureViewModelBase>());
 
         // Set up two site URLs via SelectedSites
         vm.SelectedSites.Add(new SiteInfo("https://tenant1.sharepoint.com/sites/alpha", "Alpha"));