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/en.ts

@@ -4566,6 +4566,9 @@ export default {
         minAmount: 'Minimum Amount',
         maxAmount: 'Maximum Amount',
         dailyLimit: 'Daily Limit',
+        balanceRechargeMultiplier: 'Balance Recharge Multiplier',
+        balanceRechargeMultiplierHint: 'How many USD balance the user receives for each 1 CNY paid',
+        balanceRechargePreview: 'Preview: 1 CNY = {usd} USD',
         orderTimeout: 'Order Timeout',
         orderTimeoutHint: 'In minutes, minimum 1',
         maxPendingOrders: 'Max Pending Orders',
@@ -5324,6 +5327,8 @@ export default {
   payment: {
     title: 'Recharge / Subscription',
     amountLabel: 'Amount',
+    paymentAmount: 'Payment Amount',
+    creditedBalance: 'Credited Balance',
     quickAmounts: 'Quick Amounts',
     customAmount: 'Custom Amount',
     enterAmount: 'Enter amount',
@@ -5408,6 +5413,7 @@ export default {
     amountTooLow: 'Minimum amount is {min}',
     amountTooHigh: 'Maximum amount is {max}',
     amountNoMethod: 'No payment method available for this amount',
+    rechargeRatePreview: 'Current rate: 1 CNY = {usd} USD',
     refundReason: 'Refund Reason',
     refundReasonPlaceholder: 'Please describe your refund reason',
     stripeLoadFailed: 'Failed to load payment component. Please refresh and try again.',