Refinements to drop down plans list

[jacob720]

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 #104 (comment)

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