Distances are always miles; make the unit a setting

[ssandys]

Distances in the station search results are always miles. The README lists this under Known limitations and notes it is a one-function change plus a manifest schema entry.

• the formatting lives in Model.distanceText
• Stations.search already computes distanceKm, so the underlying value is metric and the conversion is display-only
• manifest.json already has a schema array and defaults, and tests/manifest.test.js asserts that every schema key has a matching default and that Service.qml's setting() fallbacks agree with the manifest

So the guard rails for adding a setting already exist and are tested. This is the cheapest item on the enhancement list.