Switch to GNSS immediately if it turns on mid-track

If Locate was already tracking via the browser-geolocation fallback
(GNSS was off when clicked) and GNSS then gets turned on via the
sidebar, it kept using the browser position until the button was
clicked again -- the browser's own (often less accurate) position had
no business still being used once the app's own receiver had a fix.

GnssControl#onEnabledChange (a small internal callback list) notifies
LocateControl the moment the sidebar toggle changes; if it's actively
tracking via the browser at that point, it stops that and jumps to
GNSS's position instead.

Caught a real bug while verifying this: the automatic handoff reused
the same jump-to-GNSS helper a deliberate click uses, which alerts if
GNSS has no fix yet -- but GNSS only just turned on and almost
certainly hasn't gotten its first poll response back yet at the exact
moment the handoff fires, so this popped a blocking window.alert()
(which halts the whole page until dismissed) on every single enable.
A test hung on exactly this before I noticed. Fixed with an
alertIfNoFix option, true only for the deliberate-click path.

Verified end to end (simulating the "already tracking" state directly
since headless geolocation isn't reliable enough to trust as the
trigger): toggling GNSS mid-track removes the blue browser marker,
shows the green GNSS one once its poll lands, and no dialog appears.

public/js/gnss.js

@@ -15,6 +15,16 @@ export class GnssControl {
     this._marker = null;
     this._timer = null;
     this._enabled = false;
+    this._enabledListeners = [];
+  }
+
+  // Read by LocateControl: if it's mid-tracking on the browser-geolocation
+  // fallback when GNSS turns on, GNSS should take over there and then, not
+  // only the next time the button happens to get clicked — the browser's
+  // own (often less accurate) position has no business still being used
+  // once the app's own receiver has a fix.
+  onEnabledChange(callback) {
+    this._enabledListeners.push(callback);
   }
 
   mount(sidebar) {
@@ -46,6 +56,7 @@ export class GnssControl {
       this._removeMarker();
       this._status.textContent = "";
     }
+    this._enabledListeners.forEach((callback) => callback(enabled));
   }
 
   _stopPolling() {

public/js/locate.js

@@ -37,6 +37,25 @@ export class LocateControl {
     this._container = document.createElement("div");
     this._container.className = "maplibregl-ctrl maplibregl-ctrl-group";
     this._container.appendChild(this._button);
+
+    // GNSS turning on mid-track (this control's own browser-geolocation
+    // fallback, not GnssControl's — that one manages its own tracking)
+    // should switch over immediately, not just the next time this button
+    // gets clicked; the browser's own position has no business still
+    // being used once the app's own receiver has a fix. No alertIfNoFix
+    // here (unlike a deliberate click, below) — GnssControl only just
+    // turned on and almost certainly hasn't gotten its first fix back yet
+    // (its own poll is async, still in flight at the moment this fires),
+    // so this would otherwise pop a blocking "waiting for a fix" dialog on
+    // every single enable; GnssControl's own next poll will show/recenter
+    // once it actually has something, same as it would either way.
+    this._gnssControl.onEnabledChange((enabled) => {
+      if (enabled && this._active) {
+        this._stop();
+        this._jumpToGnss();
+      }
+    });
+
     return this._container;
   }
 
@@ -51,16 +70,7 @@ export class LocateControl {
 
   _click() {
     if (this._gnssControl.isEnabled()) {
-      // GNSS already follows the map on its own once enabled (GnssControl)
-      // and has no separate "off" state to toggle from here — just jump to
-      // its current fix now, for the same instant feedback clicking this
-      // button has always given.
-      const position = this._gnssControl.getPosition();
-      if (position) {
-        this._map.flyTo({ center: position, zoom: Math.max(this._map.getZoom(), 15) });
-      } else {
-        window.alert("Waiting for a GNSS fix…");
-      }
+      this._jumpToGnss({ alertIfNoFix: true });
       return;
     }
 
@@ -71,6 +81,23 @@ export class LocateControl {
     }
   }
 
+  // GNSS already follows the map on its own once enabled (GnssControl) and
+  // has no separate "off" state to toggle from here — just jump to its
+  // current fix, for the same instant feedback clicking this button has
+  // always given. alertIfNoFix is opt-in (a deliberate click wants to know
+  // why nothing happened; the automatic mid-track handoff above doesn't —
+  // see its own comment) since a JS alert() blocks the whole page until
+  // dismissed, not something to pop up on a path that isn't a direct user
+  // action.
+  _jumpToGnss({ alertIfNoFix = false } = {}) {
+    const position = this._gnssControl.getPosition();
+    if (position) {
+      this._map.flyTo({ center: position, zoom: Math.max(this._map.getZoom(), 15) });
+    } else if (alertIfNoFix) {
+      window.alert("Waiting for a GNSS fix…");
+    }
+  }
+
   _start() {
     this._active = true;
     this._button.classList.add("maplibregl-ctrl-geolocate-active");