Allow user to view computed distances in metric units.

The distance between the mapper hex and the witnessing hotspot is currently only reported in miles.

In my early prototype, I allowed toggling between miles/km by clicking on the distance cell in the table.

Objective:
* Allow user to toggle between miles/kilometers
* Save this attribute in localstorage

Start by locating the table cell that reports the mapper-to-hotspot distance and any existing browser preference handling. Implement the miles/kilometers toggle and persist the selected unit in local storage; done means the displayed distances switch correctly and the choice remains after reload.

Written by the indexing model from the issue text.