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.
