Fix possible race condition that caused a rare crash

lib/services/location_permission_gate.dart

@@ -0,0 +1,21 @@
+import 'package:geolocator/geolocator.dart';
+
+/// Single-flight guard around [Geolocator.requestPermission].
+///
+/// The Android geolocator plugin can only track one in-flight permission
+/// request at a time. If a second `requestPermission()` call is made while
+/// the system dialog from a first call is still pending, the plugin tries to
+/// reply to the same result callback twice and crashes with
+/// `IllegalStateException: Reply already submitted`.
+///
+/// We have more than one place in the app that may want to (re)check/request
+/// location permission around app startup (GPS controller, first-launch
+/// prompt). Rather than coordinate all of them, just make sure only one real
+/// native call is ever in flight - everyone else awaits that same result.
+Future<LocationPermission>? _inFlight;
+
+Future<LocationPermission> requestLocationPermissionOnce() {
+  return _inFlight ??= Geolocator.requestPermission().whenComplete(() {
+    _inFlight = null;
+  });
+}