Fix possible race condition that caused a rare crash

lib/screens/home_screen.dart

@@ -28,6 +28,7 @@ import 'coordinators/sheet_coordinator.dart';
 import 'coordinators/navigation_coordinator.dart';
 import 'coordinators/map_interaction_handler.dart';
 import 'package:geolocator/geolocator.dart';
+import '../services/location_permission_gate.dart';
 
 class HomeScreen extends StatefulWidget {
   const HomeScreen({super.key});
@@ -177,7 +178,7 @@ class _HomeScreenState extends State<HomeScreen> with TickerProviderStateMixin {
         }
 
         // Request location permission (this will show system dialog if needed)
-        final permission = await Geolocator.requestPermission();
+        final permission = await requestLocationPermissionOnce();
         debugPrint('[HomeScreen] First launch location permission result: $permission');
       }
     } catch (e) {

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;
+  });
+}

lib/widgets/map/gps_controller.dart

@@ -11,6 +11,7 @@ import '../../app_state.dart' show FollowMeMode;
 import '../../services/proximity_alert_service.dart';
 import '../../services/coordinate_validation.dart';
 import '../../services/localization_service.dart';
+import '../../services/location_permission_gate.dart';
 import '../../models/osm_node.dart';
 import '../../models/node_profile.dart';
 import '../../services/geo_bounds.dart';
@@ -152,7 +153,7 @@ class GpsController {
     }
 
     // Check permissions
-    final permission = await Geolocator.requestPermission();
+    final permission = await requestLocationPermissionOnce();
     debugPrint('[GpsController] Location permission result: $permission');
     
     switch (permission) {