Centralized health check HTML generation into a new static
`HealthCheckHtmlGenerator` class to improve modularity and
maintainability. This class encapsulates the logic for generating
HTML, including navigation, overall status, summary cards,
individual checks, and refresh information.
Replaced the inline `GenerateHealthCheckHtml` method in `Program.cs`
with the new `HealthCheckHtmlGenerator.Generate` method, removing
redundant code and improving separation of concerns. Updated
`Program.cs` to include the necessary namespace.
Simplified the `/health-ui` endpoint to use the new utility class,
reducing code duplication and improving readability.