refactor: remove API client, database, and EF Core; add ClosedXML

- Delete ApiClient/ directory (FabWorksApiClient, DTOs, interface)
- Delete Data/ and Migrations/ directories
- Add ClosedXML 0.104.2 for Excel output
- Replace FabWorksApiUrl with DefaultSuffix in app.config
- Remove .NET Framework startup config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ExportDXF/ExportDXF.csproj

@@ -14,6 +14,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="ClosedXML" Version="0.104.2" />
     <PackageReference Include="System.Configuration.ConfigurationManager" Version="8.0.0" />
   </ItemGroup>