Refinements to drop down plans list

There are a couple of visual refinement:

1. Selected state for current open sub-task
2. Background colour for the code area is `var(--mui-palette-action-hover)`. We should either not have a background, ~or use a surface or elevation for it (elevation with no shadow). The former can work for now.~

_Originally posted by @zoharma in https://github.com/DiamondLightSource/atlas/issues/104#issuecomment-4968613398_

For 2, this may not be an issue once we start using a code display component (see https://diamondlightsource.slack.com/archives/C08FER0BZN0/p1783521453670529)

Find the component that renders the dropdown plans list and inspect how it represents the currently open sub-task. Compare its code-area background with the issue screenshot and the stated styling options. Done means the open sub-task has a selected state and the code area no longer uses the action-hover background; check whether the planned code display component affects the second refinement.

Written by the indexing model from the issue text.