BGI HRMS Modern Pro Theme

Added files:
- assets/hrms/bgi-admin-modern.css
- assets/hrms/bgi-admin-modern.js
- assets/hrms/bgi-login-modern.css
- admin/hrms/theme_preview.php

Updated pages:
- admin/hrms/login.php
- admin/hrms/complete_dashboard.php
- common admin layout functions for Phase 1 to Phase 11

Features:
- Modern glass topbar
- Slide navigation menu
- Dashboard shortcut buttons
- Light/Dark theme toggle using browser localStorage
- Mobile responsive cards and tables
- Modern login page with password show/hide

No database change is required for this theme update.