feat: add test channel feature in admin UI

- POST /admin/channels/{id}/test — direct SMTP send via MailKit
- Test button shown only on existing channels (not create)
- Bootstrap modal with recipient email input and spinner
- Inline success/error result inside the modal

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Web/HrynCo.NotificationService.Web.csproj

@@ -7,6 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="MailKit" />
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Design">
       <PrivateAssets>all</PrivateAssets>