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/SharepointToolbox.Tests.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0-windows</TargetFramework>
+    <UseWPF>true</UseWPF>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="Moq" Version="4.20.72" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\SharepointToolbox\SharepointToolbox.csproj" />
+  </ItemGroup>
+
+</Project>