Pinned header at the top of the sidebar, hairlines between sections, the fold
chevron in the rows' triangle column, a count on folded sections, fainter zero
counts. From review: a move nobody could see is no move, ui.set refuses
`sidebar` together with `collapsedSidebar` and unknown names in the latter.
Translations and What's New for the sidebar work.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>