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

frontend/src/i18n/locales/zh.ts

@@ -4730,6 +4730,9 @@ export default {
         minAmount: '最低金额',
         maxAmount: '最高金额',
         dailyLimit: '每日限额',
+        balanceRechargeMultiplier: '余额充值倍率',
+        balanceRechargeMultiplierHint: '用户每支付 1 CNY 可获得多少 USD 余额',
+        balanceRechargePreview: '预览：1 CNY = {usd} USD',
         orderTimeout: '订单超时时间',
         orderTimeoutHint: '单位：分钟，至少 1 分钟',
         maxPendingOrders: '最大待支付订单数',
@@ -5512,6 +5515,8 @@ export default {
   payment: {
     title: '充值/订阅',
     amountLabel: '充值金额',
+    paymentAmount: '支付金额',
+    creditedBalance: '到账余额',
     quickAmounts: '快捷金额',
     customAmount: '自定义金额',
     enterAmount: '输入金额',
@@ -5596,6 +5601,7 @@ export default {
     amountTooLow: '最低金额为 {min}',
     amountTooHigh: '最高金额为 {max}',
     amountNoMethod: '该金额没有可用的支付方式',
+    rechargeRatePreview: '当前倍率：1 CNY = {usd} USD',
     refundReason: '退款原因',
     refundReasonPlaceholder: '请描述您的退款原因',
     stripeLoadFailed: '支付组件加载失败，请刷新页面重试',