Looks great Jan. One small issue is that for PostgreSQL I cannot specify the database when I am creating a connection. Not a problem for local databases but you may not have access to the root postgres database on a specific server eg cloud hosted database.

Apart from that, well done, I have been looking for a good db client for linux for ages. I use DBeaver at the moment, and it's pretty good, but I do find the interface a bit clunky and yours looks much better.