[Import & Export] Export and Import Contacts

[AlanaArias1297]

As an administrator, I want to export the system contacts and import new contacts from a file so that I can bulk-manage the people database.

Steps to reproduce

1. Log in as an administrator.
2. Click on the Export left navigation menu.
3. Click on the Contacts tab.
4. To export: click the Download button in the Export section.
5. To import: select a contacts file, optionally check Overwrite Existing Records, and click Upload.

Acceptance Criteria
✓ The user can navigate to the Export page.
✓ The Contacts tab shows two sections: Export and Import (upload).
✓ The Export section has a description and the Download button.
✓ The Import section shows a required Contacts file input.
✓ The 'Overwrite Existing Records' checkbox is optional; it includes explanatory help text.
✓ The Upload button is disabled while an import is in progress.
✓ An error message is shown if the import fails.
✓ On success, the contacts are updated in the system.