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.

docs/architecture.adoc

@@ -166,8 +166,8 @@ sits top-left on the map. It fits to `SLOVAKIA_BOUNDS` (a country-wide
 bounding box) rather than the plain `CENTER`/`ZOOM` point the map
 itself starts at — "home" is meant as a full-country overview, not a
 re-centering on Bratislava. It resets to that fixed default view, not
-the visitor's own location — that's `GeolocateControl`, a separate
-button in the same stack (below).
+the visitor's own location — that's `LocateControl`, a separate button
+in the same stack (below).
 
 That Home button is the top of a small top-left stack — Home, zoom
 in/out (`NavigationControl`, `showCompass: false` since this map never
@@ -175,11 +175,12 @@ rotates), a passive zoom-level readout (`ZoomLevelControl`, a plain div
 updated on the map's `zoom` event — not a button, since it has no
 click behavior of its own), a distance/area measuring tool
 (`MeasureControl`, `public/js/measure.js`), locate-me
-(`GeolocateControl`), and a "Base map" button — matching the National
-Park Service's own map control layout (grouped together on the map
-itself, rather than split across corners or between the map and a side
-panel). The last of those, `MapLayersButton` (`public/js/map.js`), is
-a second entry point into the exact same Online/Offline panel
+(`LocateControl`, `public/js/locate.js`), and a "Base map" button —
+matching the National Park Service's own map control layout (grouped
+together on the map itself, rather than split across corners or
+between the map and a side panel). The last of those,
+`MapLayersButton` (`public/js/map.js`), is a second entry point into
+the exact same Online/Offline panel
 `LayerToggleControl` already manages in the page's own sidebar — both
 call the same `LayerToggleControl#toggle()`, so there's one source of
 truth for the panel's open/closed state either way.
@@ -220,6 +221,29 @@ long any one leg was on its own. Area mode has no equivalent yet;
 a polygon's edges are its perimeter, not a path, so the same treatment
 may not even read the same way there.
 
+=== Finding your location
+
+`LocateControl` (`public/js/locate.js`) replaces MapLibre's own
+`GeolocateControl` — same button, reusing its exact CSS classes/icon
+(`.maplibregl-ctrl-geolocate`) rather than a new one, so it's a visual
+drop-in — but sourced differently. This app has its own GNSS receiver
+(gpsd, possibly `bin/gnss_simulator.rb`'s simulated one — see
+`docs/development.adoc`/`docs/deployment.adoc`), and "find my
+location" should mean that receiver first, not a device's separate
+(and often less accurate, e.g. wifi-based) location service, when the
+receiver actually has a fix. Clicking it checks
+`GET /api/gnss/position` once: if `available`, it polls that
+endpoint (same 1.5s cadence as `GnssControl`'s own sidebar polling)
+and shows the fix with the same `.gnss-marker` styling that control
+uses; if not, it falls back to the browser's own
+`navigator.geolocation.watchPosition`, GeolocateControl's own
+mechanism. Either way, only the *first* fix recenters the map —
+later updates move the marker without fighting a pan/zoom the visitor
+made in the meantime, unlike GeolocateControl's own default of
+recentering on every update. Clicking again stops tracking (clears
+the poll timer or browser watch, whichever was active) and removes
+the marker.
+
 === Debug inspector (Alt+click)
 
 Alt+click anywhere on the map opens a popup listing every rendered