QGIS PostGIS Authentication Failure with "trust" option
Just don't leave the Database field empty in the connection dialog.

When left empty, it will attempt to connect to a database named after your OS login name, which probably doesn't exist in your case.

The Host field should be left empty to use a Unix domain socket connection, as hinted by the snippet of your pg_hba.conf shown in the question.

When you put localhost in this field, it attempts a TCP connection which is not what's called local in the pg_hba.conf terminology, so that the entry local all all trust will not match with such a connection.