Keep the Community chevron under the folder tabs

The chevron on a community child's avatar had z-index 3. A chat row is not
a stacking context, so that value competed with the panels the list scrolls
under: `.chatlist-overlay` with the folder tabs sits on 3 as well, and comes
earlier in the DOM, so a row scrolled up under the tabs painted its chevron
over them. The same tie held with search, and the chevrons of the rows below
a dragged pinned row (2) went over it.

It only has to cover the avatar's own corner decorations - the premium star
and the auto-delete timer, both on 1 and earlier in the row - so 1 is enough.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>