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>

Directory.Packages.props

@@ -31,7 +31,7 @@
         <PackageVersion Include="HrynCo.Common" Version="1.0.0" />
         <PackageVersion Include="HrynCo.RabbitMq" Version="1.0.11" />
 
-        <!-- Testing -->
+        <PackageVersion Include="MailKit" Version="4.11.0" />
         <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
         <PackageVersion Include="coverlet.collector" Version="6.0.4" />
         <PackageVersion Include="xunit" Version="2.9.3" />