feat(payment): add recharge fee rate setting and fix provider card UI

- Add recharge_fee_rate system setting (percentage fee on top of recharge amount)
- Full backend chain: config constant, PaymentConfig struct, update validation,
  read/write persistence, DTO, handler GET/PUT responses
- Frontend: settings input with preview, i18n (zh/en), API types
- Fix provider card toggle layout: labels above switches to save width
- Fix Chinese translation: "EasyPay" → "易支付" in provider description

frontend/src/i18n/locales/en.ts

@@ -4569,6 +4569,9 @@ export default {
         balanceRechargeMultiplier: 'Balance Recharge Multiplier',
         balanceRechargeMultiplierHint: 'How many USD balance the user receives for each 1 CNY paid',
         balanceRechargePreview: 'Preview: 1 CNY = {usd} USD',
+        rechargeFeeRate: 'Recharge Fee Rate',
+        rechargeFeeRateHint: 'Percentage of service fee charged on top of recharge amount, 0 means no fee',
+        rechargeFeePreview: 'Preview: Recharge 100, fee {fee}',
         orderTimeout: 'Order Timeout',
         orderTimeoutHint: 'In minutes, minimum 1',
         maxPendingOrders: 'Max Pending Orders',