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() {