BaseLayerPicker now scrolls by default

The BaseLayerPicker takes up more vertical space than it used to and scrolls by default on many desktops (even when full screen). We should consider changing the styling to make it more compact or shrinking the thumbnails.

Start by locating the BaseLayerPicker component and its styling, then reproduce the scrolling behavior on a desktop-sized viewport, including fullscreen. Compare the picker’s vertical space and thumbnail sizing with the expected compact layout; done means it no longer scrolls unnecessarily while remaining usable.

Written by the indexing model from the issue text.