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

lib/models/group.dart

@@ -0,0 +1,28 @@
+import 'package:travel_mate/models/user.dart';
+
+class Group {
+  final String? id;
+  final String name;
+  final List<User> members;
+
+  Group({
+    this.id,
+    required this.name,
+    required this.members,
+  });
+
+  factory Group.fromMap(Map<String, dynamic> data, String documentId) {
+    return Group(
+      id: documentId,
+      name: data['name'] ?? '',
+      members: List<User>.from(data['members']?.map((member) => User.fromMap(member)) ?? []),
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'name': name,
+      'members': members.map((member) => member.toMap()).toList(),
+    };
+  }
+}

lib/models/message.dart

@@ -0,0 +1,13 @@
+class Message {
+  final String text; 
+  final DateTime timestamp;
+  final String senderId;
+  final String senderName;
+
+  Message({
+    required this.text,
+    required this.timestamp,
+    required this.senderId,
+    required this.senderName,
+  });
+}