sidebar: Add small refinements to the header design (#58661)

- Improve icon colors and gradient overlay
- Fix a bug where the plus icon button as visible in the sticky header
despite the menu being closed
- Made menu positining consistent between the plus/ellipsis menu
- Other small spacing/wording tweaks

Release Notes:

- N/A
dde98fa822Danilo Leal committed on 6/5/2026, 3:05:31 PM· committed by GitHubparenta8dc357
3 files changedLine totals unavailable