FeathersMenuItem does not properly show keyboard focus

bevy_feathers shows keyboard focus via a feathers.focus.ring outline, like so:

`FeathersMenuItem` fails to do so, either not showing the focus at all (in the `feathers_gallery` example, click on "Menu" and then press the down key):

or showing it by changing the background color (select the menu with tab, then press the down key) (in the screenshot, the second item is also highlighted as due to mouse hover):

Edit: Eager menus currently don't work due to a seperate bug (#24935), but the issue is visible on lazy menus.

Start with the FeathersMenuItem implementation and reproduce the issue in the feathers_gallery example using a lazy menu. Compare its keyboard-focus behavior with the feathers.focus.ring outline shown elsewhere in bevy_feathers; done means keyboard navigation displays that focus ring without incorrectly changing the background color.

Written by the indexing model from the issue text.