HTML5 Article Renderer : Adds polyfill for <dialog> to ensure browser compatibility

[AllanOXDi]

Summary

This PR adds dialog-polyfill to provide consistent behavior across all browsers.

Closes #13716

References

#13716

Reviewer guidance

Verify if :

• Polyfill is registered only when native APIs are missing.
• Lightbox opens and closes using consistent methods (showModal, close).
• Backdrop and background-inert behaviour are correctly handled.
• The escape key and backdrop click dismiss the Lightbox.
• Focus is trapped inside the dialog while open.
• Focus returns to an appropriate element after closing
• Logic doesn’t break native in supported browsers.
• Confirm no regressions in modern browsers
• Polyfill path is correctly exercised for unsupported browsers.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.19.0b2.dev0_git.65.g7417aa1d.pex
Windows Installer (EXE)	kolibri-0.19.0b2.dev0+git.65.g7417aa1d-windows-setup-unsigned.exe
Debian Package	kolibri_0.19.0b2.dev0+git.65.g7417aa1d-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.19.0b2.dev0+git.65.g7417aa1d.dmg
Android Package (APK)	kolibri-0.19.0b2.dev0+git.65.g7417aa1d-0.1.6-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.19.0b2.dev0+git.65.g7417aa1d.zip
TAR file	kolibri-0.19.0b2.dev0+git.65.g7417aa1d.tar.gz
WHL file	kolibri-0.19.0b2.dev0+git.65.g7417aa1d-py2.py3-none-any.whl