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/Services/OwnershipElevationServiceTests.cs

@@ -0,0 +1,69 @@
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.Tests.Services;
+
+[Trait("Category", "Unit")]
+public class OwnershipElevationServiceTests
+{
+    [Fact]
+    public void OwnershipElevationService_ImplementsIOwnershipElevationService()
+    {
+        var service = new OwnershipElevationService();
+        Assert.IsAssignableFrom<IOwnershipElevationService>(service);
+    }
+
+    [Fact]
+    public void AppSettings_AutoTakeOwnership_DefaultsFalse()
+    {
+        var settings = new SharepointToolbox.Core.Models.AppSettings();
+        Assert.False(settings.AutoTakeOwnership);
+    }
+
+    [Fact]
+    public async Task AppSettings_AutoTakeOwnership_RoundTripsThroughJson()
+    {
+        var json = System.Text.Json.JsonSerializer.Serialize(
+            new SharepointToolbox.Core.Models.AppSettings { AutoTakeOwnership = true },
+            new System.Text.Json.JsonSerializerOptions { PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase });
+
+        var loaded = System.Text.Json.JsonSerializer.Deserialize<SharepointToolbox.Core.Models.AppSettings>(json,
+            new System.Text.Json.JsonSerializerOptions { PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase });
+
+        Assert.NotNull(loaded);
+        Assert.True(loaded!.AutoTakeOwnership);
+    }
+
+    [Fact]
+    public void PermissionEntry_WasAutoElevated_DefaultsFalse()
+    {
+        var entry = new SharepointToolbox.Core.Models.PermissionEntry(
+            "Site", "Title", "https://example.com", false,
+            "User", "user@example.com", "Read", "Direct Permissions", "User");
+
+        Assert.False(entry.WasAutoElevated);
+    }
+
+    [Fact]
+    public void PermissionEntry_WasAutoElevated_TrueWhenSet()
+    {
+        var entry = new SharepointToolbox.Core.Models.PermissionEntry(
+            "Site", "Title", "https://example.com", false,
+            "User", "user@example.com", "Read", "Direct Permissions", "User",
+            WasAutoElevated: true);
+
+        Assert.True(entry.WasAutoElevated);
+    }
+
+    [Fact]
+    public void PermissionEntry_WithExpression_CopiesWasAutoElevated()
+    {
+        var original = new SharepointToolbox.Core.Models.PermissionEntry(
+            "Site", "Title", "https://example.com", false,
+            "User", "user@example.com", "Read", "Direct Permissions", "User");
+
+        var elevated = original with { WasAutoElevated = true };
+
+        Assert.False(original.WasAutoElevated);
+        Assert.True(elevated.WasAutoElevated);
+    }
+}