Add background color to focused spot-list-items

[b12f]

Closes https://community.openproject.org/projects/openproject/work_packages/43482/activity Closes https://community.openproject.org/projects/openproject/work_packages/43544/activity Closes https://community.openproject.org/projects/openproject/work_packages/43545/activity

[Kharonus]

Tested it, looks quite fine in the project selector. Just the question: "focusing" here means focus with our arrow key selection? ❓ Because the browser "focus" does not change background color.

I have some more questions though regarding this menu behaviour, even with those not blocking this very PR (maybe lets mention them to product team in the meeting?). This was just seen while testing it.

1. When I have a search and a nested project matches, the preselection is on the (not selectable) parent.

2. If I use tab to switch focus, the focus outline does not affect the "background selected" item, hence I get a state of two different "selected" items.

I think, those artifacts are shown by trying to put too much convenience into the component (with having the arrow key navigation while focusing the input, selection with enter while focusing the input, etc.). We should be clear, what drawbacks we are ready to take for convenience/consistency.