fix: 修复支付成功页充值金额显示 NaN

后端 ent 模型 PaymentOrder.FeeRate 带 omitempty，fee_rate=0 的订单
在 JSON 序列化时该字段会被剔除。前端 baseAmount 计算用 `fee_rate <= 0`
判断走分支，但 `undefined <= 0` 为 false，因此进入除法分支得到 NaN。

将 fee_rate 通过 Number(...) || 0 归一化，使缺失字段等价于 0。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dexcoder6

frontend/src/views/user/PaymentResultView.vue

@@ -137,13 +137,17 @@ const refreshAttempts = ref(0)
 
 /** 充值金额 = pay_amount / (1 + fee_rate/100)，fee_rate=0 时等于 pay_amount */
 const baseAmount = computed(() => {
-  if (!order.value || order.value.fee_rate <= 0) return order.value?.pay_amount ?? 0
-  return Math.round((order.value.pay_amount / (1 + order.value.fee_rate / 100)) * 100) / 100
+  if (!order.value) return 0
+  const feeRate = Number(order.value.fee_rate) || 0
+  if (feeRate <= 0) return order.value.pay_amount ?? 0
+  return Math.round((order.value.pay_amount / (1 + feeRate / 100)) * 100) / 100
 })
 
 /** 手续费 = pay_amount - baseAmount */
 const feeAmount = computed(() => {
-  if (!order.value || order.value.fee_rate <= 0) return 0
+  if (!order.value) return 0
+  const feeRate = Number(order.value.fee_rate) || 0
+  if (feeRate <= 0) return 0
   return Math.round((order.value.pay_amount - baseAmount.value) * 100) / 100
 })