Give the browser-location fallback its own (blue) marker color

The browser-geolocation fallback was reusing GnssControl's green
.gnss-marker for its own position dot -- on a machine with no real GPS
chip, wifi/IP-estimated location can jump around noticeably between
watchPosition updates, and a green dot doing that reads as "GNSS is
somehow still being used" even when it very much isn't (confirmed:
code review found no path from locate.js to /api/gnss/position at
all, and GNSS was off in the report). Switched to MapLibre's own blue
pulsing .maplibregl-user-location-dot -- the same look GeolocateControl
itself had here before, and now visually distinct from GNSS's green
on purpose rather than by accident.

Verified: both markers rendered side by side (GNSS on via the
simulator, plus a manually-placed browser-fallback-style marker) --
clearly different colors, no shared styling.

public/js/locate.js

@@ -89,10 +89,17 @@ export class LocateControl {
   // first fix, since that's the one jumping from wherever the map already
   // was; easeTo for later updates, a plain pan/zoom-preserving follow
   // rather than repeating that same swoop on every update.
+  //
+  // MapLibre's own blue pulsing "user location" dot styling
+  // (.maplibregl-user-location-dot, maplibre-gl.css — the exact look
+  // GeolocateControl itself used here before), not GnssControl's green
+  // .gnss-marker — reusing that green for a browser-estimated position too
+  // looked like this control was somehow still using GNSS data even when
+  // it very much wasn't; distinct colors for distinct sources.
   _setPosition(lon, lat) {
     if (!this._marker) {
       const el = document.createElement("div");
-      el.className = "gnss-marker";
+      el.className = "maplibregl-user-location-dot";
       this._marker = new Marker({ element: el }).setLngLat([lon, lat]).addTo(this._map);
       this._map.flyTo({ center: [lon, lat], zoom: Math.max(this._map.getZoom(), 15) });
     } else {