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>

cypress.config.ts

@@ -16,6 +16,7 @@ export default defineConfig({
       return config;
     },
     baseUrl: "http://localhost:8888",
+    scrollBehavior: "center",
     retries: {
       runMode: 2,
       openMode: 0,