refactor: separate SMTP config from LDAP struct

The LDAP struct was mixing LDAP connection settings with unrelated mail
settings. Extract mail fields into a dedicated SMTPConfig struct with
its own global (mySMTP), keeping concerns cleanly separated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ldap.go

@@ -21,11 +21,14 @@ type LDAP struct {
 	BaseDN          string
 	ServiceDN       string
 	ServicePassword string
-	MailHost        string
-	MailPort        int
-	MailUser        string
-	MailPassword    string
-	MailFrom        string
+}
+
+type SMTPConfig struct {
+	MailHost     string
+	MailPort     int
+	MailUser     string
+	MailPassword string
+	MailFrom     string
 }
 
 func (l LDAP) Connect() (*LDAPConn, error) {