feat: add Flutter mobile clients and staff delivery API

apps/service_app/lib/data/services/location_service.dart

@@ -0,0 +1,42 @@
+import 'package:geolocator/geolocator.dart';
+
+class LocationPoint {
+  const LocationPoint({
+    required this.longitude,
+    required this.latitude,
+    required this.accuracy,
+    required this.occurredAt,
+  });
+
+  final String longitude;
+  final String latitude;
+  final String accuracy;
+  final DateTime occurredAt;
+}
+
+abstract interface class LocationService {
+  Future<LocationPoint> current();
+}
+
+class GeolocatorLocationService implements LocationService {
+  @override
+  Future<LocationPoint> current() async {
+    if (!await Geolocator.isLocationServiceEnabled()) {
+      throw StateError('请先开启系统定位服务');
+    }
+    var permission = await Geolocator.checkPermission();
+    if (permission == LocationPermission.denied) {
+      permission = await Geolocator.requestPermission();
+    }
+    if (permission == LocationPermission.denied || permission == LocationPermission.deniedForever) {
+      throw StateError('定位权限未授权，无法完成该在线动作');
+    }
+    final position = await Geolocator.getCurrentPosition();
+    return LocationPoint(
+      longitude: position.longitude.toStringAsFixed(7),
+      latitude: position.latitude.toStringAsFixed(7),
+      accuracy: position.accuracy.toStringAsFixed(1),
+      occurredAt: position.timestamp,
+    );
+  }
+}