🌐 feat: add configurable USD exchange-rate support across backend & frontend

Backend
- setting/payment.go: introduce default `USDExchangeRate` (7.3)
- model/option.go:
  • inject `USDExchangeRate` into `InitOptionMap`
  • persist & sync value in `updateOptionMap`
- controller/misc.go: expose `usd_exchange_rate` via `/api/status`

Frontend
- OperationSetting.js & SettingsGeneral.js:
  • extend state/inputs with `USDExchangeRate`
  • add form field “美元汇率 (non-top-up rate, pricing only)”
- ModelPricing.js already consumes `status.usd_exchange_rate`; no change needed

API
- Administrators can update the rate via `PUT /api/option` (key: `USDExchangeRate`)
- All clients receive the latest rate through `GET /api/status`

This closes the end-to-end flow for displaying model prices in both USD and CNY based on a configurable exchange rate.

model/option.go

@@ -74,6 +74,7 @@ func InitOptionMap() {
 	common.OptionMap["EpayId"] = ""
 	common.OptionMap["EpayKey"] = ""
 	common.OptionMap["Price"] = strconv.FormatFloat(setting.Price, 'f', -1, 64)
+	common.OptionMap["USDExchangeRate"] = strconv.FormatFloat(setting.USDExchangeRate, 'f', -1, 64)
 	common.OptionMap["MinTopUp"] = strconv.Itoa(setting.MinTopUp)
 	common.OptionMap["TopupGroupRatio"] = common.TopupGroupRatio2JSONString()
 	common.OptionMap["Chats"] = setting.Chats2JsonString()
@@ -306,6 +307,8 @@ func updateOptionMap(key string, value string) (err error) {
 		setting.EpayKey = value
 	case "Price":
 		setting.Price, _ = strconv.ParseFloat(value, 64)
+	case "USDExchangeRate":
+		setting.USDExchangeRate, _ = strconv.ParseFloat(value, 64)
 	case "MinTopUp":
 		setting.MinTopUp, _ = strconv.Atoi(value)
 	case "TopupGroupRatio":