Improve accessibility of studio immersive modal

[AlexVelezLl]

Context

While testing some of the Easy Sharing of Community Channels features, I noticed that the StudioImmersiveModal was not trapping the focus, then just completed the PR with other accessibility aspects.

Summary

• Adds focus trapping for StudioImmersiveModal.
• Returns the focus to the previously focused element when the modal is closed.
• Moves the open/close handlers to a watcher of the value prop.
• Copy the focusUtils module from Kolibri to Studio.
• Add aria attributes to the modal element.

Grabacion.de.pantalla.2026-03-04.a.la.s.3.08.07.p.m.mov

Reviewer guidance

• Go to the Notifications page.
• Notice that focus is trapped correctly now.
• Check that focus returns to the previously focused element.

[MisRob]

Apologies that I missed this when this component was introduced & thanks a lot for taking care of it :)

[MisRob]

Code looks meaningful and in manual testing I haven't noticed any problems