Add a scale bar to the map

MapLibre's own ScaleControl, bottom-left (attribution already owns
bottom-right) — no custom NPS-style bar needed, the built-in one covers it.

public/js/map.js

@@ -2,6 +2,7 @@ import {
   Map as MaplibreMap,
   NavigationControl,
   Popup,
+  ScaleControl,
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
 import { GpxPanelControl } from "/js/gpx.js";
@@ -427,6 +428,7 @@ const gnssControl = new GnssControl(map);
 // the "GNSS" sidebar icon actually turns it on — not just whenever a
 // receiver happens to have a fix. See locate.js.
 map.addControl(new LocateControl(gnssControl), "top-left");
+map.addControl(new ScaleControl({ unit: "metric" }), "bottom-left");
 
 // A small inset map showing where the current view sits in context — NPS's
 // own overview plugin (docs/nps-resources.adoc's "overview" entry, still on