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.Tests/ViewModels/SettingsViewModelOwnershipTests.cs

@@ -0,0 +1,64 @@
+using System.IO;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
+using SharepointToolbox.Infrastructure.Persistence;
+using SharepointToolbox.Services;
+using SharepointToolbox.ViewModels;
+using SharepointToolbox.ViewModels.Tabs;
+
+namespace SharepointToolbox.Tests.ViewModels;
+
+[Trait("Category", "Unit")]
+public class SettingsViewModelOwnershipTests : IDisposable
+{
+    private readonly string _tempFile;
+
+    public SettingsViewModelOwnershipTests()
+    {
+        _tempFile = Path.GetTempFileName();
+        File.Delete(_tempFile);
+    }
+
+    public void Dispose()
+    {
+        if (File.Exists(_tempFile)) File.Delete(_tempFile);
+        if (File.Exists(_tempFile + ".tmp")) File.Delete(_tempFile + ".tmp");
+    }
+
+    private SettingsViewModel CreateViewModel()
+    {
+        var settingsService = new SettingsService(new SettingsRepository(_tempFile));
+        var mockBranding = new Mock<IBrandingService>().Object;
+        var logger = NullLogger<FeatureViewModelBase>.Instance;
+        return new SettingsViewModel(settingsService, mockBranding, logger);
+    }
+
+    [Fact]
+    public async Task LoadAsync_AutoTakeOwnership_LoadsFalseByDefault()
+    {
+        var vm = CreateViewModel();
+
+        await vm.LoadAsync();
+
+        Assert.False(vm.AutoTakeOwnership);
+    }
+
+    [Fact]
+    public async Task SetAutoTakeOwnership_True_CallsSetAutoTakeOwnershipAsync()
+    {
+        // Use a real SettingsService backed by temp file to verify persistence
+        var settingsService = new SettingsService(new SettingsRepository(_tempFile));
+        var mockBranding = new Mock<IBrandingService>().Object;
+        var logger = NullLogger<FeatureViewModelBase>.Instance;
+        var vm = new SettingsViewModel(settingsService, mockBranding, logger);
+
+        await vm.LoadAsync();
+        vm.AutoTakeOwnership = true;
+
+        // Small delay to let the fire-and-forget persist
+        await Task.Delay(100);
+
+        var persisted = await settingsService.GetSettingsAsync();
+        Assert.True(persisted.AutoTakeOwnership);
+    }
+}