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>

src/components/LayerEditor.tsx

@@ -370,7 +370,7 @@ class LayerEditorInternal extends React.Component<LayerEditorInternalProps, Laye
         aria-label={t("Layer editor")}
         data-wd-key="layer-editor"
       >
-        <header>
+        <header data-wd-key="layer-editor.header">
           <div className="layer-header">
             <h2 className="layer-header__title">
               {t("Layer: {{layerId}}", { layerId: formatLayerId(this.props.layer.id) })}

src/components/LayerList.tsx

@@ -289,7 +289,7 @@ class LayerListContainerInternal extends React.Component<LayerListContainerInter
         onOpenToggle={this.toggleModal.bind(this, "add")}
         onLayersChange={this.props.onLayersChange}
       />
-      <header className="maputnik-layer-list-header">
+      <header className="maputnik-layer-list-header" data-wd-key="layer-list.header">
         <span className="maputnik-layer-list-header-title">{t("Layers")}</span>
         <span className="maputnik-space" />
         <div className="maputnik-default-property">