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',

frontend/src/i18n/locales/zh.ts

@@ -4726,13 +4726,16 @@ export default {
         enabledHint: '启用或禁用支付系统',
         enabledPaymentTypes: '启用的服务商',
         enabledPaymentTypesHint: '禁用服务商将同时禁用对应的实例。',
-        findProvider: '正在寻找合适的 EasyPay 服务商？',
+        findProvider: '正在寻找合适的易支付服务商？',
         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',
+        rechargeFeeRate: '充值手续费率',
+        rechargeFeeRateHint: '用户充值时额外收取的手续费百分比，0 表示不收取手续费',
+        rechargeFeePreview: '预览：充值 100 元，手续费 {fee} 元',
         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',