Fix SMTP configuration to use Cloudron sendmail hostname

Change SMTP host from localhost/127.0.0.1 to 'mail' as per Cloudron
sendmail addon documentation. The sendmail addon provides a local SMTP
relay accessible via hostname 'mail' on port 2525.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

start.sh

@@ -122,7 +122,7 @@ HASH_KEY_FILE="$SECRETS_DIR/hash_key"
 JWT_SECRET_FILE="$SECRETS_DIR/jwt_secret"
 SESSION_SECRET_FILE="$SECRETS_DIR/session_secret"
 
-SMTP_HOST="localhost"
+SMTP_HOST="mail"
 SMTP_PORT="2525"
 SMTP_ENCRYPTION=""
 SMTP_USERNAME=""