Add a distance/area measuring tool

A ruler button in the top-left control stack, matching NPS's own
Area/Distance ruler. Click to add vertices; distance sums haversine
segment lengths, area uses the spherical-excess polygon formula
(same one turf.js's area() uses) rather than a flat shoelace, since a
flat approximation drifts at park-boundary scale. SI units only
(m/km, m²/km²) — no imperial toggle.

public/js/map.js

@@ -10,6 +10,7 @@ import { GpxPanelControl } from "/js/gpx.js";
 import { GnssControl } from "/js/gnss.js";
 import { LiveTrackPanelControl } from "/js/livetrack.js";
 import { TrackProfilePanel } from "/js/track_profile.js";
+import { MeasureControl } from "/js/measure.js";
 import { LAYERS_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import HomeButton from "/vendor/npmap-home-button/home-button.esm.js";
@@ -228,8 +229,9 @@ const map = new MaplibreMap({
   hash: "map",
 });
 
-// A compact top-left stack — Home, zoom, zoom-level readout, locate-me,
-// base map — matching the National Park Service's own map control layout,
+// A compact top-left stack — Home, zoom, zoom-level readout, measure,
+// locate-me, base map — matching the National Park Service's own map
+// control layout,
 // all grouped together rather than split across corners. Added in this
 // order since MapLibre stacks same-position controls in add order.
 //
@@ -245,6 +247,7 @@ map.addControl(new HomeButton({ homePosition: CENTER, zoom: ZOOM }), "top-left")
 // (NavigationControl's default third button) would just be dead weight.
 map.addControl(new NavigationControl({ showCompass: false }), "top-left");
 map.addControl(new ZoomLevelControl(), "top-left");
+map.addControl(new MeasureControl(), "top-left");
 map.addControl(
   new GeolocateControl({
     positionOptions: { enableHighAccuracy: true },