feat: add customer session refresh functionality and improve session management

- Introduced RealtimeCustomerSessionRefreshPayload and RealtimeCustomerSessionRefreshEvent types for handling session refresh events.
- Updated ws_service to verify customer session and handle session refresh notifications.
- Enhanced API client to manage customer session tokens and expiration.
- Implemented customer session validation and storage in session storage.
- Added functions to exchange and ensure customer sessions.
- Updated IM real-time connection to include customer session tokens in WebSocket requests.
- Modified SDK and widget configurations to support external IDs and user tokens.

web/lib/kefu-widget-config.ts

@@ -4,9 +4,9 @@ export type KefuWidgetHostConfig = {
   apiBaseUrl?: string
   /** 外部访客稳定标识；未传时使用浏览器本地访客 ID */
   externalId?: string
-  /** 访客展示名，随请求以 X-External-Name / WS query externalName 传给后端 */
+  /** 访客展示名，仅用于首次换取客服会话 token */
   externalName?: string
-  /** 业务系统签发的前台用户 JWT */
+  /** 业务系统签发的前台用户 JWT，仅用于首次换取客服会话 token */
   userToken?: string
   title?: string
   subtitle?: string