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_statistics.dart

@@ -0,0 +1,25 @@
+class GroupStatistics {
+  final double totalExpenses;
+  final int expenseCount;
+  final double averageExpense;
+  final String topCategory;
+  final Map<String, double> categoryBreakdown;
+
+  const GroupStatistics({
+    required this.totalExpenses,
+    required this.expenseCount,
+    required this.averageExpense,
+    required this.topCategory,
+    required this.categoryBreakdown,
+  });
+
+  factory GroupStatistics.empty() {
+    return const GroupStatistics(
+      totalExpenses: 0.0,
+      expenseCount: 0,
+      averageExpense: 0.0,
+      topCategory: '',
+      categoryBreakdown: {},
+    );
+  }
+}