feat: Add GPS toggle and node sorting

lib/services/relay_candidate_sorter.dart

@@ -0,0 +1,48 @@
+import 'package:geolocator/geolocator.dart';
+import 'package:latlong2/latlong.dart';
+
+import '../models/contact.dart';
+
+class RelayCandidateSorter {
+  const RelayCandidateSorter();
+
+  List<Contact> sortByDistanceFromSelf(
+    List<Contact> contacts, {
+    required LatLng? selfPoint,
+  }) {
+    final sorted = List<Contact>.from(contacts);
+    sorted.sort((a, b) {
+      if (selfPoint != null) {
+        final distanceCompare = _distanceFrom(
+          selfPoint,
+          a,
+        ).compareTo(_distanceFrom(selfPoint, b));
+        if (distanceCompare != 0) {
+          return distanceCompare;
+        }
+      }
+
+      final nameCompare = a.displayName.compareTo(b.displayName);
+      if (nameCompare != 0) {
+        return nameCompare;
+      }
+
+      return a.publicKeyHex.compareTo(b.publicKeyHex);
+    });
+    return sorted;
+  }
+
+  double _distanceFrom(LatLng selfPoint, Contact contact) {
+    final location = contact.displayLocation;
+    if (location == null) {
+      return double.infinity;
+    }
+
+    return Geolocator.distanceBetween(
+      selfPoint.latitude,
+      selfPoint.longitude,
+      location.latitude,
+      location.longitude,
+    );
+  }
+}