feat: add Airwallex payments and multi-currency support

backend/internal/payment/amount.go

@@ -1,24 +1,9 @@
 package payment
 
-import (
-	"fmt"
-
-	"github.com/shopspring/decimal"
-)
-
-const centsPerYuan = 100
-
-// YuanToFen converts a CNY yuan string (e.g. "10.50") to fen (int64).
-// Uses shopspring/decimal for precision.
 func YuanToFen(yuanStr string) (int64, error) {
-	d, err := decimal.NewFromString(yuanStr)
-	if err != nil {
-		return 0, fmt.Errorf("invalid amount: %s", yuanStr)
-	}
-	return d.Mul(decimal.NewFromInt(centsPerYuan)).IntPart(), nil
+	return AmountToMinorUnit(yuanStr, DefaultPaymentCurrency)
 }
 
-// FenToYuan converts fen (int64) to yuan as a float64 for interface compatibility.
 func FenToYuan(fen int64) float64 {
-	return decimal.NewFromInt(fen).Div(decimal.NewFromInt(centsPerYuan)).InexactFloat64()
+	return MinorUnitToAmount(fen, DefaultPaymentCurrency)
 }