fix(outline): Make sure the outline is always in foreground

[mejo-]

📝 Summary

Add a border shadow around the outline. Also, make it overlay the editor content (e.g. on small screens). Required e.g. for Collectives.

Fixes: #4276

Copied from #4945 (review):

Having the outline floating without borders was part of the design requirements or feedback when we introduced it, so I'd not change that as part of a bugfix at least.

@nextcloud/designers for feedback from the design team.

🖼️ Screenshots

🏚️ Before Text standalone	🏡 After Text standalone

🏚️ Before Collectives	🏡 After Collectives

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits

[marcoambrosini]

I think the outline should definitely go in the right sidebar. We cannot add yet another column to collectives (we have 4).

[nimishavijay]

Agreed with Marco, in Collectives it could go in the sidebar.
In Text, I do still think that we could keep the outline without borders, it blends seamlessly with the rest of the document that way. Otherwise it feels a bit like losing the immersiveness of a full screen text editor, what do you think? :)

[mejo-]

Let's discuss this separately for collectives. The text related parts are already addressed.