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/expense_split.dart

@@ -0,0 +1,58 @@
+import 'package:equatable/equatable.dart';
+
+class ExpenseSplit extends Equatable {
+  final String userId;
+  final String userName;
+  final double amount;
+  final bool isPaid;
+  final DateTime? paidAt;
+
+  const ExpenseSplit({
+    required this.userId,
+    required this.userName,
+    required this.amount,
+    this.isPaid = false,
+    this.paidAt,
+  });
+
+  factory ExpenseSplit.fromMap(Map<String, dynamic> map) {
+    return ExpenseSplit(
+      userId: map['userId'] ?? '',
+      userName: map['userName'] ?? '',
+      amount: (map['amount'] as num?)?.toDouble() ?? 0.0,
+      isPaid: map['isPaid'] ?? false,
+      paidAt: map['paidAt'] != null 
+          ? DateTime.fromMillisecondsSinceEpoch(map['paidAt'])
+          : null,
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'userId': userId,
+      'userName': userName,
+      'amount': amount,
+      'isPaid': isPaid,
+      'paidAt': paidAt?.millisecondsSinceEpoch,
+    };
+  }
+
+  ExpenseSplit copyWith({
+    String? userId,
+    String? userName,
+    double? amount,
+    bool? isPaid,
+    DateTime? paidAt,
+  }) {
+    return ExpenseSplit(
+      userId: userId ?? this.userId,
+      userName: userName ?? this.userName,
+      amount: amount ?? this.amount,
+      isPaid: isPaid ?? this.isPaid,
+      paidAt: paidAt ?? this.paidAt,
+    );
+  }
+
+  @override
+  List<Object?> get props => [userId, userName, amount, isPaid];
+}