feat: Add channel datagrams #1928

ref:

test/services/location_tracking_service_test.dart

@@ -0,0 +1,40 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:meshcore_sar_app/services/location_tracking_service.dart';
+import 'package:meshcore_sar_app/services/profiles_feature_service.dart';
+
+void main() {
+  final service = LocationTrackingService();
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    ProfileStorageScope.setScope(
+      profilesEnabled: false,
+      activeProfileId: 'default',
+    );
+    service.fastLocationUpdatesEnabled = false;
+    service.fastLocationMovementThresholdMeters = 10.0;
+    service.fastLocationActiveCadenceSeconds = 10;
+    service.fastLocationChannelIdx = null;
+  });
+
+  test('persists and restores fast location channel idx', () async {
+    await service.updateFastLocationChannelIdx(3);
+
+    service.fastLocationChannelIdx = null;
+    await service.loadSettings();
+
+    expect(service.fastLocationChannelIdx, 3);
+  });
+
+  test('clears persisted fast location channel idx when unset', () async {
+    await service.updateFastLocationChannelIdx(7);
+    await service.updateFastLocationChannelIdx(null);
+
+    service.fastLocationChannelIdx = 99;
+    await service.loadSettings();
+
+    expect(service.fastLocationChannelIdx, isNull);
+  });
+}