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/api/admin/settings.ts

@@ -126,6 +126,7 @@ export interface SystemSettings {
   payment_enabled_types: string[]
   payment_balance_disabled: boolean
   payment_balance_recharge_multiplier: number
+  payment_recharge_fee_rate: number
   payment_load_balance_strategy: string
   payment_product_name_prefix: string
   payment_product_name_suffix: string
@@ -233,6 +234,7 @@ export interface UpdateSettingsRequest {
   payment_enabled_types?: string[]
   payment_balance_disabled?: boolean
   payment_balance_recharge_multiplier?: number
+  payment_recharge_fee_rate?: number
   payment_load_balance_strategy?: string
   payment_product_name_prefix?: string
   payment_product_name_suffix?: string