💱 feat(settings): introduce site-wide quota display type (USD/CNY/TOKENS/CUSTOM)

Replace the legacy boolean “DisplayInCurrencyEnabled” with an injected, type-safe
configuration `general_setting.quota_display_type`, and wire it through the
backend and frontend.

Backend
- Add `QuotaDisplayType` to `operation_setting.GeneralSetting` with injected
  registration via `config.GlobalConfig.Register("general_setting", ...)`.
  Helpers: `IsCurrencyDisplay()`, `IsCNYDisplay()`, `GetQuotaDisplayType()`.
- Expose `quota_display_type` in `/api/status` and keep legacy
  `display_in_currency` for backward compatibility.
- Logger: update `LogQuota` and `FormatQuota` to support USD/CNY/TOKENS. When
  CNY is selected, convert using `operation_setting.USDExchangeRate`.
- Controllers:
  - `billing`: compute subscription/usage amounts based on the selected type
    (USD: divide by `QuotaPerUnit`; CNY: USD→CNY; TOKENS: keep raw tokens).
  - `topup` / `topup_stripe`: treat inputs as “amount” for USD/CNY and as
    token-count for TOKENS; adjust min topup and pay money accordingly.
  - `misc`: include `quota_display_type` in status payload.
- Compatibility: in `model/option.UpdateOption`, map updates to
  `DisplayInCurrencyEnabled` → `general_setting.quota_display_type`
  (true→USD, false→TOKENS). Keep exporting the legacy key in `OptionMap`.

Frontend
- Settings: replace the “display in currency” switch with a Select
  (`general_setting.quota_display_type`) offering USD / CNY / Tokens.
  Provide fallback mapping from legacy `DisplayInCurrencyEnabled`.
- Persist `quota_display_type` to localStorage (keep `display_in_currency`
  for legacy components).
- Rendering helpers: base all quota/price rendering on `quota_display_type`;
  use `usd_exchange_rate` for CNY symbol/values.
- Pricing page: default view currency follows site display type (USD/CNY),
  while TOKENS mode still allows per-view currency toggling when needed.

Notes
- No database migrations required.
- Legacy clients remain functional via compatibility fields.

controller/billing.go

@@ -5,6 +5,7 @@ import (
 	"one-api/common"
 	"one-api/dto"
 	"one-api/model"
+	"one-api/setting/operation_setting"
 )
 
 func GetSubscription(c *gin.Context) {
@@ -39,8 +40,18 @@ func GetSubscription(c *gin.Context) {
 	}
 	quota := remainQuota + usedQuota
 	amount := float64(quota)
-	if common.DisplayInCurrencyEnabled {
-		amount /= common.QuotaPerUnit
+	// OpenAI 兼容接口中的 *_USD 字段含义保持“额度单位”对应值：
+	// 我们将其解释为以“站点展示类型”为准：
+	// - USD: 直接除以 QuotaPerUnit
+	// - CNY: 先转 USD 再乘汇率
+	// - TOKENS: 直接使用 tokens 数量
+	switch operation_setting.GetQuotaDisplayType() {
+	case operation_setting.QuotaDisplayTypeCNY:
+		amount = amount / common.QuotaPerUnit * operation_setting.USDExchangeRate
+	case operation_setting.QuotaDisplayTypeTokens:
+		// amount 保持 tokens 数值
+	default:
+		amount = amount / common.QuotaPerUnit
 	}
 	if token != nil && token.UnlimitedQuota {
 		amount = 100000000
@@ -80,8 +91,13 @@ func GetUsage(c *gin.Context) {
 		return
 	}
 	amount := float64(quota)
-	if common.DisplayInCurrencyEnabled {
-		amount /= common.QuotaPerUnit
+	switch operation_setting.GetQuotaDisplayType() {
+	case operation_setting.QuotaDisplayTypeCNY:
+		amount = amount / common.QuotaPerUnit * operation_setting.USDExchangeRate
+	case operation_setting.QuotaDisplayTypeTokens:
+		// tokens 保持原值
+	default:
+		amount = amount / common.QuotaPerUnit
 	}
 	usage := OpenAIUsageResponse{
 		Object:     "list",