Update nerve cuff selection to be consistent with other selectable objects

[napakalas]

This update makes the nerve cuff visually and behaviorally consistent with other selectable objects in the map:

• The nerve cuff now has a blue background with a dashed grey border when inactive.
• Selecting a nerve highlights its inner elements and other objects in the connected paths.
• Selecting a path correctly highlights the corresponding nerves.

[dbrnz]

All looks good, thanks!