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/Core/Models/AppSettings.cs

@@ -4,4 +4,5 @@ public class AppSettings
 {
     public string DataFolder { get; set; } = string.Empty;
     public string Lang { get; set; } = "en";
+    public bool AutoTakeOwnership { get; set; } = false;
 }