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/Localization/TranslationSourceTests.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Tests.Localization;
+
+public class TranslationSourceTests
+{
+    [Fact(Skip = "Wave 0 stub — implemented in plan 01-05")]
+    public void ChangeLanguage_Updates_Bound_Strings() { }
+}