feat: Implement group management features with Firestore integration and loading states

lib/services/group_service.dart

@@ -0,0 +1,26 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:travel_mate/models/group.dart';
+
+class GroupService {
+  final FirebaseFirestore _firestore = FirebaseFirestore.instance;
+
+  Stream<List<Group>> getGroupsStream() {
+    return _firestore.collection('groups').snapshots().map((snapshot) {
+      return snapshot.docs.map((doc) {
+        return Group.fromMap(doc.data(), doc.id);
+      }).toList();
+    });
+  }
+
+  Future<void> createGroup(Group group) async {
+    await _firestore.collection('groups').add(group.toMap());
+  }
+
+  Future<void> updateGroup(Group group) async {
+    await _firestore.collection('groups').doc(group.id).update(group.toMap());
+  }
+
+  Future<void> deleteGroup(String groupId) async {
+    await _firestore.collection('groups').doc(groupId).delete();
+  }
+}