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/SettingsService.cs

@@ -36,4 +36,11 @@ public class SettingsService
         settings.DataFolder = path;
         await _repository.SaveAsync(settings);
     }
+
+    public async Task SetAutoTakeOwnershipAsync(bool enabled)
+    {
+        var settings = await _repository.LoadAsync();
+        settings.AutoTakeOwnership = enabled;
+        await _repository.SaveAsync(settings);
+    }
 }