I was in the irc when this was initially discussed. Correct me if I'm wrong, @xPaw.
I believe there isn't one being used, and this was the issue?
We noticed that cloudflare (maybe others as well) doesn't allow calls to sites without a user agent, so I believe that this means we can't get some site information for the thumbnails
In general, it's best to use a custom user-agent, as most of the existing ones would be browsers, which we don't really want to pretend to be (actually, maybe we do? Because the request comes from the browser itself, so maybe we should use the user's browser's user agent)
Something like
Shout/v0.0.0 (https://github.com/erming/shout)
would work.