feat: cumulative balance with carried_over in finance summary

internal/repository/finance.go

@@ -237,15 +237,34 @@ func (r *FinanceRepository) DeleteTransaction(id, userID int64) error {
 func (r *FinanceRepository) GetSummary(userID int64, month, year int) (*model.FinanceSummary, error) {
 	summary := &model.FinanceSummary{}
 
+	// First day of selected month and last day of selected month
+	firstDay := time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)
+	lastDay := firstDay.AddDate(0, 1, -1)
+
+	// Carried over: balance of all transactions BEFORE the selected month
+	err := r.db.QueryRow(`SELECT COALESCE(SUM(CASE WHEN type='income' THEN amount ELSE -amount END), 0)
+		FROM finance_transactions WHERE user_id=$1 AND date < $2`,
+		userID, firstDay).Scan(&summary.CarriedOver)
+	if err != nil {
+		return nil, err
+	}
+
 	// Total income & expense for the month
-	err := r.db.QueryRow(`SELECT COALESCE(SUM(CASE WHEN type='income' THEN amount ELSE 0 END),0),
+	err = r.db.QueryRow(`SELECT COALESCE(SUM(CASE WHEN type='income' THEN amount ELSE 0 END),0),
 		COALESCE(SUM(CASE WHEN type='expense' THEN amount ELSE 0 END),0)
 		FROM finance_transactions WHERE user_id=$1 AND EXTRACT(MONTH FROM date)=$2 AND EXTRACT(YEAR FROM date)=$3`,
 		userID, month, year).Scan(&summary.TotalIncome, &summary.TotalExpense)
 	if err != nil {
 		return nil, err
 	}
-	summary.Balance = summary.TotalIncome - summary.TotalExpense
+
+	// Cumulative balance: all transactions up to end of selected month
+	err = r.db.QueryRow(`SELECT COALESCE(SUM(CASE WHEN type='income' THEN amount ELSE -amount END), 0)
+		FROM finance_transactions WHERE user_id=$1 AND date <= $2`,
+		userID, lastDay).Scan(&summary.Balance)
+	if err != nil {
+		return nil, err
+	}
 
 	// By category
 	rows, err := r.db.Query(`SELECT c.id, c.name, c.emoji, t.type, SUM(t.amount) as amount