💱 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/topup.go

@@ -86,8 +86,9 @@ func GetEpayClient() *epay.Client {
 
 func getPayMoney(amount int64, group string) float64 {
 	dAmount := decimal.NewFromInt(amount)
-
-	if !common.DisplayInCurrencyEnabled {
+	// 充值金额以“展示类型”为准：
+	// - USD/CNY: 前端传 amount 为金额单位；TOKENS: 前端传 tokens，需要换成 USD 金额
+	if operation_setting.GetQuotaDisplayType() == operation_setting.QuotaDisplayTypeTokens {
 		dQuotaPerUnit := decimal.NewFromFloat(common.QuotaPerUnit)
 		dAmount = dAmount.Div(dQuotaPerUnit)
 	}
@@ -115,7 +116,7 @@ func getPayMoney(amount int64, group string) float64 {
 
 func getMinTopup() int64 {
 	minTopup := operation_setting.MinTopUp
-	if !common.DisplayInCurrencyEnabled {
+	if operation_setting.GetQuotaDisplayType() == operation_setting.QuotaDisplayTypeTokens {
 		dMinTopup := decimal.NewFromInt(int64(minTopup))
 		dQuotaPerUnit := decimal.NewFromFloat(common.QuotaPerUnit)
 		minTopup = int(dMinTopup.Mul(dQuotaPerUnit).IntPart())
@@ -176,7 +177,7 @@ func RequestEpay(c *gin.Context) {
 		return
 	}
 	amount := req.Amount
-	if !common.DisplayInCurrencyEnabled {
+	if operation_setting.GetQuotaDisplayType() == operation_setting.QuotaDisplayTypeTokens {
 		dAmount := decimal.NewFromInt(int64(amount))
 		dQuotaPerUnit := decimal.NewFromFloat(common.QuotaPerUnit)
 		amount = dAmount.Div(dQuotaPerUnit).IntPart()