Add Staff fix

Reason for the error shown in Backoffice > Add Staff:
- The live users table includes legacy/required columns such as company_id, staff_code, and full_name.
- The previous Add Staff page only inserted employee_code, first_name, last_name, email, phone, password_hash, role_id, and is_active.
- MySQL rejected the insert, but the page displayed the generic duplicate email/employee code message.

Fix included:
- backoffice/staff-add.php now writes company_id, staff_code, full_name, and employee_code where those columns exist.
- backoffice/staff-edit.php keeps staff_code, full_name, and employee_code in sync.
- backoffice/config.php has schema-aware helpers so this works with the current live database without adding new tables.
- Duplicate email/employee/staff code checks now show a more specific message before insert/update.

Upload/extract this ZIP inside /home/uent/public_html.
