Add automatic Vite build to .NET Host project

Build Target: BuildFrontend
- Installs pnpm dependencies
- Builds frontend with Vite
- Copies dist to wwwroot

Result: dotnet publish includes frontend automatically

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/KArtSell.Host.csproj

@@ -1,5 +1,19 @@
-<Project Sdk="Microsoft.NET.Sdk.Web"><PropertyGroup>  <UserSecretsId>bab7e095-067e-4797-b2ab-df4c1f8b447d</UserSecretsId>
+<Project Sdk="Microsoft.NET.Sdk.Web">
+  <PropertyGroup>
+    <UserSecretsId>bab7e095-067e-4797-b2ab-df4c1f8b447d</UserSecretsId>
   </PropertyGroup>
+
+  <!-- Frontend Build Target: Automatically build Vite and copy to wwwroot -->
+  <Target Name="BuildFrontend" BeforeTargets="Build">
+    <Exec Command="pnpm install --frozen-lockfile" WorkingDirectory="$(ProjectDir)../../frontend" ContinueOnError="false" />
+    <Exec Command="pnpm build" WorkingDirectory="$(ProjectDir)../../frontend" ContinueOnError="false" />
+    <Copy SourceFiles="@(FrontendFiles)" DestinationFolder="$(ProjectDir)wwwroot/%(RecursiveDir)" />
+  </Target>
+
+  <ItemGroup>
+    <FrontendFiles Include="../../frontend/dist/**/*" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="../KArtSell.BuildingBlocks/KArtSell.BuildingBlocks.csproj" />
     <ProjectReference Include="../KArtSell.Modules.SignalEngine/KArtSell.Modules.SignalEngine.csproj" />