fix(dovecot): use %n instead of %u for username and home path

Update the Dovecot authentication template to use `%n` (user part only)
instead of `%u` (full username) in both the `passdb` username format and
the `userdb` home directory path.

This ensures that user lookups and home directory generation rely specifically
on the username portion, excluding the domain part.

templates/auth-dovecot-users.conf.ext.j2

@@ -3,10 +3,10 @@
 
 passdb {
   driver = passwd-file
-  args = scheme=SHA512-CRYPT username_format=%u /etc/dovecot/users
+  args = scheme=SHA512-CRYPT username_format=%n /etc/dovecot/users
 }
 
 userdb {
   driver = static
-  args = uid=vmail gid=vmail home=/var/vmail/%u
+  args = uid=vmail gid=vmail home=/var/vmail/%n
 }