fix: keep headers visible when scrolling left panes (#1485)

Keeps headers visible when scrolling left panes as described in the
[issue 951
](https://github.com/maplibre/maputnik/issues/951)

The fix was manually confirmed to be working, see video below.

## Before (taken from the issue)
<img width="713" height="231" alt="image"
src="https://github.com/user-attachments/assets/c1eadb0d-6dbf-4199-8732-68b07d626003"
/>


## After

https://github.com/user-attachments/assets/ab5e4a6f-c5f9-44fd-850d-8eac58c35c68

---------

Co-authored-by: Harel M <harel.mazor@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -22,6 +22,7 @@
 - Fix missing spec info when clicking next to a property
 - Fix Firefox open file that stopped working due to react upgrade
 - Fix issue with missing bottom error panel
+- Fixed headers in left panes (Layers list and Layer editor) to remain visible when scrolling
 - _...Add new stuff here..._
 
 ## 3.0.0