💱 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/misc.go

@@ -64,18 +64,22 @@ func GetStatus(c *gin.Context) {
 		"top_up_link":                 common.TopUpLink,
 		"docs_link":                   operation_setting.GetGeneralSetting().DocsLink,
 		"quota_per_unit":              common.QuotaPerUnit,
-		"display_in_currency":         common.DisplayInCurrencyEnabled,
-		"enable_batch_update":         common.BatchUpdateEnabled,
-		"enable_drawing":              common.DrawingEnabled,
-		"enable_task":                 common.TaskEnabled,
-		"enable_data_export":          common.DataExportEnabled,
-		"data_export_default_time":    common.DataExportDefaultTime,
-		"default_collapse_sidebar":    common.DefaultCollapseSidebar,
-		"mj_notify_enabled":           setting.MjNotifyEnabled,
-		"chats":                       setting.Chats,
-		"demo_site_enabled":           operation_setting.DemoSiteEnabled,
-		"self_use_mode_enabled":       operation_setting.SelfUseModeEnabled,
-		"default_use_auto_group":      setting.DefaultUseAutoGroup,
+		// 兼容旧前端：保留 display_in_currency，同时提供新的 quota_display_type
+		"display_in_currency":           operation_setting.IsCurrencyDisplay(),
+		"quota_display_type":            operation_setting.GetQuotaDisplayType(),
+		"custom_currency_symbol":        operation_setting.GetGeneralSetting().CustomCurrencySymbol,
+		"custom_currency_exchange_rate": operation_setting.GetGeneralSetting().CustomCurrencyExchangeRate,
+		"enable_batch_update":           common.BatchUpdateEnabled,
+		"enable_drawing":                common.DrawingEnabled,
+		"enable_task":                   common.TaskEnabled,
+		"enable_data_export":            common.DataExportEnabled,
+		"data_export_default_time":      common.DataExportDefaultTime,
+		"default_collapse_sidebar":      common.DefaultCollapseSidebar,
+		"mj_notify_enabled":             setting.MjNotifyEnabled,
+		"chats":                         setting.Chats,
+		"demo_site_enabled":             operation_setting.DemoSiteEnabled,
+		"self_use_mode_enabled":         operation_setting.SelfUseModeEnabled,
+		"default_use_auto_group":        setting.DefaultUseAutoGroup,
 
 		"usd_exchange_rate": operation_setting.USDExchangeRate,
 		"price":             operation_setting.Price,