feat: Add GPS toggle and node sorting

test/services/relay_candidate_sorter_test.dart

@@ -0,0 +1,60 @@
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:latlong2/latlong.dart';
+
+import 'package:meshcore_sar_app/models/contact.dart';
+import 'package:meshcore_sar_app/services/relay_candidate_sorter.dart';
+
+void main() {
+  test('sorts relay candidates by distance from self point', () {
+    final sorter = RelayCandidateSorter();
+
+    final sorted = sorter.sortByDistanceFromSelf([
+      _contact(seed: 1, name: 'Far', lat: 46.08, lon: 14.60),
+      _contact(seed: 2, name: 'Near', lat: 46.0570, lon: 14.5060),
+      _contact(seed: 3, name: 'Middle', lat: 46.06, lon: 14.52),
+    ], selfPoint: const LatLng(46.0569, 14.5058));
+
+    expect(sorted.map((contact) => contact.displayName).toList(), [
+      'Near',
+      'Middle',
+      'Far',
+    ]);
+  });
+
+  test('falls back to stable name ordering when self point is unavailable', () {
+    final sorter = RelayCandidateSorter();
+
+    final sorted = sorter.sortByDistanceFromSelf([
+      _contact(seed: 1, name: 'Zulu', lat: 46.08, lon: 14.60),
+      _contact(seed: 2, name: 'Alpha', lat: 46.0570, lon: 14.5060),
+    ], selfPoint: null);
+
+    expect(sorted.map((contact) => contact.displayName).toList(), [
+      'Alpha',
+      'Zulu',
+    ]);
+  });
+}
+
+Contact _contact({
+  required int seed,
+  required String name,
+  required double lat,
+  required double lon,
+}) {
+  final publicKey = Uint8List(32)..fillRange(0, 32, seed);
+  return Contact(
+    publicKey: publicKey,
+    type: ContactType.repeater,
+    flags: 0,
+    outPathLen: -1,
+    outPath: Uint8List(64),
+    advName: name,
+    lastAdvert: DateTime.now().millisecondsSinceEpoch ~/ 1000,
+    advLat: (lat * 1e6).round(),
+    advLon: (lon * 1e6).round(),
+    lastMod: DateTime.now().millisecondsSinceEpoch ~/ 1000,
+  );
+}