feat(payment): balance recharge multiplier and refund amount separation

- Add balance_recharge_multiplier system setting (e.g. 1.2 = charge 100 get 120)
- Separate order_amount (credited balance) from pay_amount (actual payment)
- Refund calculates gateway amount proportionally from pay_amount
- Frontend shows both amounts in order details, payment status, refund dialog
- Admin settings UI for configuring recharge multiplier

backend/internal/service/payment_amounts.go

@@ -0,0 +1,37 @@
+package service
+
+import (
+	"math"
+
+	"github.com/shopspring/decimal"
+)
+
+const defaultBalanceRechargeMultiplier = 1.0
+
+func normalizeBalanceRechargeMultiplier(multiplier float64) float64 {
+	if math.IsNaN(multiplier) || math.IsInf(multiplier, 0) || multiplier <= 0 {
+		return defaultBalanceRechargeMultiplier
+	}
+	return multiplier
+}
+
+func calculateCreditedBalance(paymentAmount, multiplier float64) float64 {
+	return decimal.NewFromFloat(paymentAmount).
+		Mul(decimal.NewFromFloat(normalizeBalanceRechargeMultiplier(multiplier))).
+		Round(2).
+		InexactFloat64()
+}
+
+func calculateGatewayRefundAmount(orderAmount, payAmount, refundAmount float64) float64 {
+	if orderAmount <= 0 || payAmount <= 0 || refundAmount <= 0 {
+		return 0
+	}
+	if math.Abs(refundAmount-orderAmount) <= amountToleranceCNY {
+		return decimal.NewFromFloat(payAmount).Round(2).InexactFloat64()
+	}
+	return decimal.NewFromFloat(payAmount).
+		Mul(decimal.NewFromFloat(refundAmount)).
+		Div(decimal.NewFromFloat(orderAmount)).
+		Round(2).
+		InexactFloat64()
+}