feat(18-01): models, SettingsService, OwnershipElevationService + tests

- AppSettings.AutoTakeOwnership bool property defaulting to false
- PermissionEntry.WasAutoElevated optional param (default false, last position)
- SettingsService.SetAutoTakeOwnershipAsync persists toggle
- IOwnershipElevationService interface + OwnershipElevationService wrapping Tenant.SetSiteAdmin
- SettingsViewModel.AutoTakeOwnership property loads and persists via SetAutoTakeOwnershipAsync
- DI registration in App.xaml.cs (Phase 18 section)
- 8 new tests: models, persistence, service, viewmodel

SharepointToolbox/Services/IOwnershipElevationService.cs

@@ -0,0 +1,8 @@
+using Microsoft.SharePoint.Client;
+
+namespace SharepointToolbox.Services;
+
+public interface IOwnershipElevationService
+{
+    Task ElevateAsync(ClientContext tenantAdminCtx, string siteUrl, string loginName, CancellationToken ct);
+}