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/App.xaml.cs

@@ -161,6 +161,9 @@ public partial class App : Application
         // Phase 17: Group Expansion
         services.AddTransient<ISharePointGroupResolver, SharePointGroupResolver>();
 
+        // Phase 18: Auto-Take Ownership
+        services.AddTransient<IOwnershipElevationService, OwnershipElevationService>();
+
         // Phase 7: User Access Audit
         services.AddTransient<IUserAccessAuditService, UserAccessAuditService>();
         services.AddTransient<IGraphUserSearchService, GraphUserSearchService>();