Move global usings from GlobalUsings.cs files to .csproj files

- Convert GlobalUsings.cs files to <Using> ItemGroup in project files
- Remove GlobalUsings.cs files from both LeetCode.CSharp and LeetCode projects
- Maintain same global imports functionality using MSBuild <Using> elements
- Build, tests, and formatting all pass successfully

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: si618

LeetCode.CSharp/GlobalUsings.cs

@@ -5,6 +5,18 @@
     <RootNamespace>LeetCode.CSharp</RootNamespace>
   </PropertyGroup>
 
+  <ItemGroup>
+    <Using Include="BenchmarkDotNet.Attributes" />
+    <Using Include="BenchmarkDotNet.Columns" />
+    <Using Include="BenchmarkDotNet.Configs" />
+    <Using Include="BenchmarkDotNet.Diagnosers" />
+    <Using Include="LeetCode.CSharp.Problems" />
+    <Using Include="Shouldly" />
+    <Using Include="System.Diagnostics.CodeAnalysis" />
+    <Using Include="System.Text" />
+    <Using Include="Xunit" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="BenchmarkDotNet" />
     <PackageReference Include="coverlet.collector" />

LeetCode.CSharp/LeetCode.CSharp.csproj

@@ -6,6 +6,26 @@
         <OutputType>Exe</OutputType>
     </PropertyGroup>
 
+    <ItemGroup>
+      <Using Include="BenchmarkDotNet.Loggers" />
+      <Using Include="BenchmarkDotNet.Reports" />
+      <Using Include="BenchmarkDotNet.Running" />
+      <Using Include="LeetCode.Commands" />
+      <Using Include="LeetCode.CSharp" />
+      <Using Include="LeetCode.Menus" />
+      <Using Include="LeetCode.Menus.Selections" />
+      <Using Include="Lolcat" />
+      <Using Include="Spectre.Console" />
+      <Using Include="Spectre.Console.Cli" />
+      <Using Include="Spectre.Console.Rendering" />
+      <Using Include="System.ComponentModel" />
+      <Using Include="System.Diagnostics.CodeAnalysis" />
+      <Using Include="System.Reflection" />
+      <Using Include="System.Text" />
+      <Using Include="System.Text.Json.Nodes" />
+      <Using Include="System.Text.RegularExpressions" />
+    </ItemGroup>
+
     <ItemGroup>
       <PackageReference Include="BenchmarkDotNet" />
       <PackageReference Include="lolcat" />