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

backend/internal/handler/dto/settings.go

@@ -137,6 +137,7 @@ type SystemSettings struct {
 	PaymentEnabledTypes              []string `json:"payment_enabled_types"`
 	PaymentBalanceDisabled           bool     `json:"payment_balance_disabled"`
 	PaymentBalanceRechargeMultiplier float64  `json:"payment_balance_recharge_multiplier"`
+	PaymentRechargeFeeRate              float64  `json:"payment_recharge_fee_rate"`
 	PaymentLoadBalanceStrat          string   `json:"payment_load_balance_strategy"`
 	PaymentProductNamePrefix string   `json:"payment_product_name_prefix"`
 	PaymentProductNameSuffix string   `json:"payment_product_name_suffix"`