Prefer this app's own GNSS receiver for "find my location"

LocateControl (public/js/locate.js) replaces MapLibre's own
GeolocateControl -- same button, reusing its exact CSS/icon so it's a
visual drop-in, but sourced differently. Checks GET /api/gnss/position
once on click: if a fix is available (gpsd, possibly
bin/gnss_simulator.rb's simulated one), tracks that instead of the
browser's separate (and often less accurate) location service, which
is now only a fallback. Only the first fix recenters the map; later
updates just move the marker, unlike GeolocateControl's own default
of recentering on every update, which would fight a pan/zoom made in
the meantime.

Verified end to end with the GNSS simulator running: clicking the
button centered on the simulated position and dropped a marker,
clicking again cleared both, no console errors.

public/js/map.js

@@ -1,7 +1,6 @@
 import {
   Map as MaplibreMap,
   NavigationControl,
-  GeolocateControl,
   Popup,
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
@@ -11,6 +10,7 @@ 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 { LocateControl } from "/js/locate.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";
@@ -349,7 +349,7 @@ const map = new MaplibreMap({
 // above), "current view" and "the default view" are no longer the same
 // thing, and a home button that resets to wherever you happened to load
 // the page wouldn't be a home button. This is *not* "go to my location" —
-// that's GeolocateControl, below.
+// that's LocateControl, below.
 const SLOVAKIA_BOUNDS = [
   [16.83, 47.73],
   [22.57, 49.61],
@@ -360,14 +360,12 @@ map.addControl(new HomeButton({ bounds: SLOVAKIA_BOUNDS }), "top-left");
 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 },
-    trackUserLocation: true,
-    showAccuracyCircle: true,
-  }),
-  "top-left",
-);
+// Not MapLibre's own GeolocateControl (browser Geolocation API only) —
+// this app has its own GNSS receiver (gpsd, possibly a simulated one, see
+// bin/gnss_simulator.rb) and "find my location" should prefer that over a
+// device's separate, often less accurate location service when it has a
+// fix. See locate.js.
+map.addControl(new LocateControl(), "top-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