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/payment_handler.go

@@ -132,6 +132,7 @@ func (h *PaymentHandler) GetCheckoutInfo(c *gin.Context) {
 		Plans:                     planList,
 		BalanceDisabled:           cfg.BalanceDisabled,
 		BalanceRechargeMultiplier: cfg.BalanceRechargeMultiplier,
+		RechargeFeeRate:           cfg.RechargeFeeRate,
 		HelpText:                  cfg.HelpText,
 		HelpImageURL:              cfg.HelpImageURL,
 		StripePublishableKey:      cfg.StripePublishableKey,
@@ -145,6 +146,7 @@ type checkoutInfoResponse struct {
 	Plans                     []checkoutPlan                  `json:"plans"`
 	BalanceDisabled           bool                            `json:"balance_disabled"`
 	BalanceRechargeMultiplier float64                         `json:"balance_recharge_multiplier"`
+	RechargeFeeRate           float64                         `json:"recharge_fee_rate"`
 	HelpText                  string                          `json:"help_text"`
 	HelpImageURL              string                          `json:"help_image_url"`
 	StripePublishableKey      string                          `json:"stripe_publishable_key"`