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_member.dart

@@ -0,0 +1,51 @@
+class GroupMember {
+  final String userId;
+  final String firstName;
+  final String pseudo; // Pseudo du membre (par défaut = prénom)
+  final String role; // 'admin' ou 'member'
+  final DateTime joinedAt;
+
+  GroupMember({
+    required this.userId,
+    required this.firstName,
+    String? pseudo,
+    this.role = 'member',
+    DateTime? joinedAt,
+  })  : pseudo = pseudo ?? firstName, // Par défaut, pseudo = prénom
+        joinedAt = joinedAt ?? DateTime.now();
+
+  factory GroupMember.fromMap(Map<String, dynamic> map, String userId) {
+    return GroupMember(
+      userId: userId,
+      firstName: map['firstName'] ?? '',
+      pseudo: map['pseudo'] ?? map['firstName'] ?? '',
+      role: map['role'] ?? 'member',
+      joinedAt: DateTime.fromMillisecondsSinceEpoch(map['joinedAt'] ?? 0),
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'firstName': firstName,
+      'pseudo': pseudo,
+      'role': role,
+      'joinedAt': joinedAt.millisecondsSinceEpoch,
+    };
+  }
+
+  GroupMember copyWith({
+    String? userId,
+    String? firstName,
+    String? pseudo,
+    String? role,
+    DateTime? joinedAt,
+  }) {
+    return GroupMember(
+      userId: userId ?? this.userId,
+      firstName: firstName ?? this.firstName,
+      pseudo: pseudo ?? this.pseudo,
+      role: role ?? this.role,
+      joinedAt: joinedAt ?? this.joinedAt,
+    );
+  }
+}