💱 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.

web/src/i18n/locales/fr.json

@@ -1773,7 +1773,10 @@
   "自定义模型名称": "Nom de modèle personnalisé",
   "启用全部密钥": "Activer toutes les clés",
   "充值价格显示": "Prix de recharge",
-  "美元汇率（非充值汇率，仅用于定价页面换算）": "Taux de change USD (pas de taux de recharge, uniquement utilisé pour la conversion de la page de tarification)",
+  "站点额度展示类型及汇率": "Type d'affichage du quota du site et taux de change",
+  "自定义货币": "Devise personnalisée",
+  "自定义货币符号": "Symbole de devise personnalisé",
+  "例如 €, £, Rp, ₩, ₹...": "Par exemple, €, £, Rp, ₩, ₹...",
   "美元汇率": "Taux de change USD",
   "隐藏操作项": "Masquer les actions",
   "显示操作项": "Afficher les actions",
@@ -2137,4 +2140,4 @@
   "common": {
     "changeLanguage": "Changer de langue"
   }
-}
+}