Make "find my location" actually follow GNSS, gated on the sidebar toggle

Two fixes based on feedback after the last commit:

1. The map wasn't following the position at all -- only the first fix
   recentered, later updates just moved the marker. Now every fix
   recenters (easeTo, not flyTo, after the first one, so it doesn't
   repeat that zoom swoop on every update) -- matches how
   GeolocateControl's own trackUserLocation always behaved.

2. GNSS should only be used when explicitly turned on via the sidebar
   "GNSS" icon, not whenever a receiver happens to have a fix --
   simpler to test against the simulator that way, and puts the
   visitor in control of whether GNSS is engaged at all. GnssControl
   now owns following the map on its own (same flyTo-once/easeTo-after
   pattern) whenever it's enabled; LocateControl checks
   GnssControl#isEnabled() and, when true, just jumps to
   #getPosition()'s current fix and reuses its existing .gnss-marker
   instead of polling/marking on its own. Only falls back to the
   browser's own geolocation when GNSS is off.

Verified end to end with the GNSS simulator: enabling sidebar GNSS
recenters immediately and keeps following over several polls;
clicking Locate while GNSS is on jumps to the same position without
creating a second marker or touching its own active state; turning
GNSS back off and clicking Locate correctly falls through to the
browser geolocation path instead.

public/js/map.js

@@ -360,12 +360,18 @@ 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");
+// Constructed here (rather than down where it's mounted, alongside the
+// other sidebar controls) so LocateControl can hold the same instance —
+// its constructor only touches `map`, mount() (which needs the sidebar
+// element) can still happen later.
+const gnssControl = new GnssControl(map);
 // 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");
+// bin/gnss_simulator.rb), and "find my location" should prefer that over a
+// device's separate, often less accurate location service, but only once
+// 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");
 
 // 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
@@ -398,7 +404,7 @@ const profilePanel = new TrackProfilePanel(map);
 profilePanel.mount();
 new GpxPanelControl(map, profilePanel).mount(sidebar);
 new RadarControl(map).mount(sidebar);
-new GnssControl(map).mount(sidebar);
+gnssControl.mount(sidebar);
 new LiveTrackPanelControl(map).mount(sidebar);
 
 function applyStyle(name) {