Admin hierarchy update for Mistake / Warning

New hierarchy:
Admin > CSR Manager > Team Leader > Agent

What changed:
- Added Admin Mistake page: admin/mistake.php
- Added Admin Warning page: admin/warning.php
- Added Admin sidebar links for Mistake and Warning
- Admin can send Mistake / Warning to CSR Managers, Team Leaders, and Agents
- CSR Manager can still send only to Team Leaders and Agents
- Team Leader can still send only to Agents inside their own team
- Agents can only view and mark their own Mistake / Warning as read
- Records sent by Admin now display "Admin" correctly in the recipient's Mistake / Warning page

Database:
- No new SQL import is required if the previous Mistake / Warning update already created csr_discipline_records.
- The code still auto-creates the discipline table if missing.
