Add dashed nerve cuff representation to the map legend

[napakalas]

Now that the nerve cuff is represented as a dashed circle, its appearance should be reflected in the map legend.

In flatmap-viewer, this is handled using the attribute:

borderStyle: 'dashed'

However, flatmapvuer currently renders all borders as solid, regardless of the borderStyle value. The rendering logic needs to be updated so that dashed borders are displayed correctly in the legend.

See the attached image for the expected legend appearance.