Fix settings link in Arc popup

[ddhjy]

Summary

• open the settings page through an explicit extension URL instead of runtime.openOptionsPage()
• route the popup settings button through the existing openSettings message
• prevent the popup settings anchor from falling back to #

Why

In Arc, clicking the popup settings button from a non-clippable page such as arc://extensions/ can land on the extension error page instead of opening the Web Clipper settings page. Opening settings.html?section=general explicitly avoids Arc's openOptionsPage() behavior.

Testing

• npx webpack --env BROWSER=chrome --mode development
• loaded the unpacked extension in Arc and verified the popup settings button opens settings.html?section=general