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/components/payment/ToggleSwitch.vue

@@ -1,6 +1,6 @@
 <template>
-  <label class="flex items-center gap-1.5 cursor-pointer">
-    <span class="text-xs text-gray-500 dark:text-gray-400">{{ label }}</span>
+  <label class="flex flex-col items-center gap-0.5 cursor-pointer">
+    <span class="text-xs text-gray-500 dark:text-gray-400 whitespace-nowrap">{{ label }}</span>
     <button
       type="button"
       role="switch"