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/ViewModels/Tabs/SettingsViewModel.cs

@@ -39,6 +39,19 @@ public partial class SettingsViewModel : FeatureViewModelBase
         }
     }
 
+    private bool _autoTakeOwnership;
+    public bool AutoTakeOwnership
+    {
+        get => _autoTakeOwnership;
+        set
+        {
+            if (_autoTakeOwnership == value) return;
+            _autoTakeOwnership = value;
+            OnPropertyChanged();
+            _ = _settingsService.SetAutoTakeOwnershipAsync(value);
+        }
+    }
+
     private string? _mspLogoPreview;
     public string? MspLogoPreview
     {
@@ -65,8 +78,10 @@ public partial class SettingsViewModel : FeatureViewModelBase
         var settings = await _settingsService.GetSettingsAsync();
         _selectedLanguage = settings.Lang;
         _dataFolder = settings.DataFolder;
+        _autoTakeOwnership = settings.AutoTakeOwnership;
         OnPropertyChanged(nameof(SelectedLanguage));
         OnPropertyChanged(nameof(DataFolder));
+        OnPropertyChanged(nameof(AutoTakeOwnership));
 
         var mspLogo = await _brandingService.GetMspLogoAsync();
         MspLogoPreview = mspLogo is not null ? $"data:{mspLogo.MimeType};base64,{mspLogo.Base64}" : null;