Enhance the user interface and functionality for applying signatures:
- Add detailed display of signature fields and their pages.
- Introduce a progress bar to show marking progress visually.
- Display success message when all fields are marked.
- Automatically submit signatures when all fields are marked.
- Provide guidance for creating a signature if none exists.
- Disable "Export Signed PDF" button until signature is applied.
- Refactor code to simplify logic and remove redundancy.
These changes improve usability and provide clearer feedback to users.