try to do better

backend/src/services/expense_service.rs

@@ -150,11 +150,12 @@ impl ExpenseService {
 
         let mut result: Vec<MonthlyExpenseGroup> = grouped
             .into_iter()
-            .map(|((year, month), expenses)| {
+            .map(|((year, month), mut expenses)| {
                 let total_amount: Decimal = expenses
                     .iter()
                     .map(|e| e.amount)
                     .sum();
+                expenses.sort_by(|a, b| b.created_at.cmp(&a.created_at));
                 MonthlyExpenseGroup {
                     year,
                     month,