HRMS - Passport Photo & Image Preview Update

1. Employee Documents now include a required Passport Size Photo (JPG/JPEG/PNG).
2. Manager/Area Manager document-upload flow includes Passport Size Photo.
3. Admin direct employee creation includes Passport Size Photo.
4. Passport photo is stored in both employees.passport_photo and employee_requests.passport_photo.
5. Existing employee document images are now displayed as real thumbnails in View Employee -> Documents.
6. Clicking an image opens a large preview popup. PDF documents continue to open in a new tab.
7. Existing installations are automatically migrated by dashboard.php; no manual ALTER TABLE is required.
8. database.sql and schema.sql were updated for fresh installations.

Important: Passport Size Photo is required for new direct Admin employee creation and for the document-upload stage before final acceptance.