sidebar: Display a new thread button in the archive view (#59114)

Sometimes, landing in the archive view at first, particularly after
importing ACP threads, can be confusing as there's not an immediately
available "create new thread" button users can click on.

Release Notes:

- N/A
1c3806a7a1Danilo Leal committed on 6/11/2026, 2:17:01 PM· committed by GitHubparentd130d03
2 files changedLine totals unavailable