use headless renderwindow for CILViewer via trame

[paskino]

Describe your changes

Pass headless variable to the web application to force headless render window.

Describe any testing you have performed

Consider adding example code to examples

Start the web app as

python web_app.py --headless /path/to/files

Link relevant issues

• Closes CILViewer via trame requires headless renderwindow #428

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the CIL developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'waiting for review'

Contribution Notes

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CILViewer (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties

Qt contributions should follow Qt naming conventions i.e. camelCase method names.

VTK contributions should follow VTK naming conventions i.e. PascalCase method names.

[DanicaSTFC]

@paskino I will not include this in the v25.0.0 release. Let's aim to close this for the successive release, which will be after the iDVC workshop in Bath - May 2025.