feat: bepusdt支付渠道增加钱包地址显示和复制功能

- 后端: 添加wallet_address字段到支付信息返回
- 前端: Checkout页面显示钱包地址，支持点击复制

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/api/handlers/payment.go

@@ -156,6 +156,7 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 	tradeID, _ := data["trade_id"].(string)
 	token, _ := data["token"].(string)
 	paymentURL, _ := data["payment_url"].(string)
+	walletAddress, _ := data["address"].(string)
 
 	var amount float64
 	if v, ok := data["amount"].(float64); ok {
@@ -207,6 +208,7 @@ func (h *PaymentHandler) CreatePayment(c *gin.Context) {
 		"actual_amount":   actualAmount,
 		"token":           token,
 		"payment_url":     paymentURL,
+		"wallet_address":  walletAddress,
 		"expiration_time": remainingSeconds,
 		"trade_type":      tradeType,
 		"channel_id":      channel.ID,