refactor: extract BLE protocol stack into meshcore_client package

Move BLE communication layer (command queue, frame parser/builder,
protocol constants, data models) into a standalone reusable Dart package
at ../meshcore_client. App model files become thin re-export wrappers,
keeping all existing consumers working without import changes.

lib/models/advert_location.dart

@@ -1,40 +1 @@
-import 'package:latlong2/latlong.dart';
-
-/// Single advertisement location point in a contact's movement history
-class AdvertLocation {
-  final LatLng location;
-  final DateTime timestamp;
-
-  AdvertLocation({
-    required this.location,
-    required this.timestamp,
-  });
-
-  /// Get friendly time ago display
-  String get timeAgo {
-    final diff = DateTime.now().difference(timestamp);
-    if (diff.inMinutes < 1) return 'Just now';
-    if (diff.inMinutes < 60) return '${diff.inMinutes}m ago';
-    if (diff.inHours < 24) return '${diff.inHours}h ago';
-    return '${diff.inDays}d ago';
-  }
-
-  @override
-  String toString() {
-    return 'AdvertLocation(lat: ${location.latitude.toStringAsFixed(6)}, '
-           'lon: ${location.longitude.toStringAsFixed(6)}, '
-           'time: ${timestamp.toIso8601String()})';
-  }
-
-  @override
-  bool operator ==(Object other) {
-    if (identical(this, other)) return true;
-    return other is AdvertLocation &&
-           other.location.latitude == location.latitude &&
-           other.location.longitude == location.longitude &&
-           other.timestamp == timestamp;
-  }
-
-  @override
-  int get hashCode => Object.hash(location.latitude, location.longitude, timestamp);
-}
+export 'package:meshcore_client/meshcore_client.dart' show AdvertLocation;