feat(external-imap): 新增外部 IMAP 全局开关与后台配置

- 支持在系统设置中启用/禁用外部 IMAP，并持久化相关密钥与 OAuth 配置。
- 在公共设置和前端页面中联动开关状态，未启用时隐藏外部 IMAP 相关功能并拦截后端接口。
- 补充外部 IMAP 默认关闭、配置说明与回调地址文档。

apps/web/src/lib/api-types.ts

@@ -172,10 +172,18 @@ export type SystemSettings = {
   userMailboxApplyEnabled: boolean
   userMailboxDomainIds: string[]
   reservedMailboxPrefixes: string
+  externalImapEnabled: boolean
+  externalImapSecretSet: boolean
+  externalImapSyncSeconds: number
+  externalImapAllowPrivateHosts: boolean
+  externalImapGmailClientId: string
+  externalImapGmailClientSecretSet: boolean
+  externalImapOutlookClientId: string
+  externalImapOutlookClientSecretSet: boolean
 }
-export type SystemSettingsPayload = Omit<SystemSettings, "smtpPasswordSet" | "turnstileSecretSet"> & { smtpPassword: string; turnstileSecretKey: string }
+export type SystemSettingsPayload = Omit<SystemSettings, "smtpPasswordSet" | "turnstileSecretSet" | "externalImapSecretSet" | "externalImapGmailClientSecretSet" | "externalImapOutlookClientSecretSet"> & { smtpPassword: string; turnstileSecretKey: string; externalImapSecretKey: string; externalImapGmailClientSecret: string; externalImapOutlookClientSecret: string }
 export type PublicDomain = { id: string; name: string }
-export type PublicSettings = { openRegistration: boolean; turnstileEnabled: boolean; turnstileSiteKey: string; publicHostname: string; mailAutoRefresh: boolean; mailRefreshMs: number; mailboxDomains?: PublicDomain[] }
+export type PublicSettings = { openRegistration: boolean; turnstileEnabled: boolean; turnstileSiteKey: string; publicHostname: string; mailAutoRefresh: boolean; mailRefreshMs: number; externalImapEnabled: boolean; mailboxDomains?: PublicDomain[] }
 export type LoginPayload = { email?: string; password?: string; turnstileToken?: string; challengeToken?: string; twoFactorCode?: string }
 export type LoginResponse = { user?: User; twoFactorRequired?: boolean; challengeToken?: string }
 export type RegisterPayload = { email: string; displayName: string; password: string; turnstileToken?: string; domainId?: string; localPart?: string }