🌐 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.

controller/misc.go

@@ -77,6 +77,7 @@ func GetStatus(c *gin.Context) {
 		"self_use_mode_enabled":    operation_setting.SelfUseModeEnabled,
 		"default_use_auto_group":   setting.DefaultUseAutoGroup,
 		"pay_methods":              setting.PayMethods,
+		"usd_exchange_rate":        setting.USDExchangeRate,
 
 		// 面板启用开关
 		"api_info_enabled":      cs.ApiInfoEnabled,