Dovecot is configured to listen for auth requests at private/auth
:
unix_listener /var/spool/postfix/private/auth
However, you're telling Postfix to connect to an entirely different location:
smtpd_sasl_path = smtpd
– in fact, you're (almost) telling it to send the auth requests to its own SMTP daemon, which isn't going to understand them at all, much less forward them to Dovecot.
Change the SASL socket path in Postfix to private/auth
.