feat: configure standard special-use mailboxes in dovecot

Update templates/10-mail.conf.j2 to explicitly define standard mailboxes (Drafts, Junk, Trash, Sent) within the inbox namespace.

This change ensures that:
- These folders are automatically subscribed (`auto = subscribe`).
- They are correctly flagged with `special_use` attributes, improving compatibility with IMAP clients.
- "Sent Messages" is also recognized as a Sent folder for broader client support.

templates/10-mail.conf.j2

@@ -4,4 +4,24 @@
 mail_location = {{ dovecot_mail_location }}
 namespace inbox {
   inbox = yes
+
+  mailbox Drafts {
+    special_use = \Drafts
+    auto = subscribe
+  }
+  mailbox Junk {
+    special_use = \Junk
+    auto = subscribe
+  }
+  mailbox Trash {
+    special_use = \Trash
+    auto = subscribe
+  }
+  mailbox Sent {
+    special_use = \Sent
+    auto = subscribe
+  }
+  mailbox "Sent Messages" {
+    special_use = \Sent
+  }
 }