I’m not giving access to my Mastodon account to some random service I’ve never heard of for no reason.

If it makes you feel better, it's all client-side: there's nothing executing on the server (I'm running a copy of it on a server that just... can't execute anything) so it's not doing any data stealing.

Buuut, since it's trivial to host, you could grab a copy of the code and host it yourself as well.