feat: Add User and UserBalance models with serialization methods

feat: Implement BalanceRepository for group balance calculations

feat: Create ExpenseRepository for managing expenses

feat: Add services for handling expenses and storage operations

fix: Update import paths for models in repositories and services

refactor: Rename CountContent to AccountContent in HomePage

chore: Add StorageService for image upload and management

lib/models/group.dart

@@ -0,0 +1,65 @@
+import 'group_member.dart';
+
+class Group {
+  final String id; 
+  final String name;
+  final String tripId;
+  final String createdBy;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+  final List<GroupMember> members;
+
+  Group({
+    required this.id, 
+    required this.name,
+    required this.tripId,
+    required this.createdBy,
+    DateTime? createdAt,
+    DateTime? updatedAt,
+    List<GroupMember>? members,
+  })  : createdAt = createdAt ?? DateTime.now(),
+        updatedAt = updatedAt ?? DateTime.now(),
+        members = members ?? [];
+
+  factory Group.fromMap(Map<String, dynamic> map, String id) {
+    return Group(
+      id: id,
+      name: map['name'] ?? '',
+      tripId: map['tripId'] ?? '',
+      createdBy: map['createdBy'] ?? '',
+      createdAt: DateTime.fromMillisecondsSinceEpoch(map['createdAt'] ?? 0),
+      updatedAt: DateTime.fromMillisecondsSinceEpoch(map['updatedAt'] ?? 0),
+      members: [],
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'name': name,
+      'tripId': tripId,
+      'createdBy': createdBy,
+      'createdAt': createdAt.millisecondsSinceEpoch,
+      'updatedAt': updatedAt.millisecondsSinceEpoch,
+    };
+  }
+
+  Group copyWith({
+    String? id,
+    String? name,
+    String? tripId,
+    String? createdBy,
+    DateTime? createdAt,
+    DateTime? updatedAt,
+    List<GroupMember>? members,
+  }) {
+    return Group(
+      id: id ?? this.id,
+      name: name ?? this.name,
+      tripId: tripId ?? this.tripId,
+      createdBy: createdBy ?? this.createdBy,
+      createdAt: createdAt ?? this.createdAt,
+      updatedAt: updatedAt ?? this.updatedAt,
+      members: members ?? this.members,
+    );
+  }
+}