Allow readers to change Catalog

I remember us discussing this a very long time ago and while I think we wanted to do it, I can't find a related ticket.

The idea is to allow reader users to set an arbitrary Catalog URL instead of https://opds.library.kiwix.org one.

There are UX challenges as it needs to maybe point to server-setup instructions (`kiwix-serve` must be launched with `--contentServerURL`) and it must properly handle that URL not working or not having download links and inform the user correctly and allow him to easily switch back to the Kiwix Catalog.

Is this still something we want to do?

Note: this was requested on the desk today.

Start by locating how the reader selects and stores its Catalog URL, then review the kiwix-serve --contentServerURL setup requirement. Define the behavior for unreachable catalogs, catalogs without download links, setup guidance, and switching back to the Kiwix Catalog; the issue does not name files or tests.

Written by the indexing model from the issue text.