Fix open link with relativ urls

If a page https://example.org contains html like <a href="/link-to- other-post-on-this-site">click here</a> QDesktopServices::openUrl tries to open "/link-to-other-post-on-this-site" as a local file.

This patch adds the base url and the scheme from the page link for relative urls to make them work.

You can test it with the first link here https://volkerkrause.eu/2025/05/31/kde-itinerary-april-may-2025.html