feat(01-01): add xUnit test project with 7 stub test files

- SharepointToolbox.Tests targeting net10.0-windows with UseWPF=true
- Moq 4.20.72 added for future mocking
- 7 stub test files across Services, Auth, ViewModels, Localization, Integration
- All tests marked [Fact(Skip)] referencing implementation plan — 0 failed, 7 skipped
- Solution build: 0 errors, 0 warnings

SharepointToolbox.Tests/Services/ProfileServiceTests.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Tests.Services;
+
+public class ProfileServiceTests
+{
+    [Fact(Skip = "Wave 0 stub — implemented in plan 01-03")]
+    public void SaveAndLoad_RoundTrips_Profiles() { }
+}

SharepointToolbox.Tests/Services/SettingsServiceTests.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Tests.Services;
+
+public class SettingsServiceTests
+{
+    [Fact(Skip = "Wave 0 stub — implemented in plan 01-03")]
+    public void SaveAndLoad_RoundTrips_Settings() { }
+}