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/account.dart

@@ -0,0 +1,54 @@
+import 'group_member.dart';
+
+class Account {
+  final String id;
+  final String tripId;
+  final String groupId;
+  final String name;
+  final List<GroupMember> members;
+
+  Account({
+    required this.id,
+    required this.tripId,
+    required this.groupId,
+    required this.name,
+    List<GroupMember>? members,
+  }) : members = members ?? [];
+
+
+  factory Account.fromMap(Map<String, dynamic> map) {
+    return Account(
+      id: map['id'] as String,
+      tripId: map['tripId'] as String,
+      groupId: map['groupId'] as String,
+      name: map['name'] as String,
+      members: [],
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'id': id,
+      'tripId': tripId,
+      'groupId': groupId,
+      'name': name,
+      'members': members.map((member) => member.toMap()).toList(),
+    };
+  }
+
+  Account copyWith({
+    String? id,
+    String? tripId,
+    String? groupId,
+    String? name,
+    List<GroupMember>? members,
+  }) {
+    return Account(
+      id: id ?? this.id,
+      tripId: tripId ?? this.tripId,
+      groupId: groupId ?? this.groupId,
+      name: name ?? this.name,
+      members: members ?? this.members,
+    );
+  }
+}