[Bug]: Expose does not work after a while

### Platform

Linux

### Operating system version

Ubuntu 24.04

### System architecture

Intel (x86)

### PolyScope Version

0.17

### Bug description

After a couple of hours or the next day, Expose does not work.
Also, the APP_URL is red from the master polyscope.json and not the branch polyscope.json.

### Steps to reproduce

1. Start the preview
2. The next day, the preview does not work. Clicking on browse again does not initiate a new expose session.

### Relevant log output

```shell

```

首先比较 master 和 branch 的 polyscope.json 文件中的 APP_URL 值，然后跟踪从启动 preview、点击 Browse 到发起 Expose 会话的整个 preview 流程。在 Ubuntu 24.04 上复现第二天出现的故障，并使用相关日志确定 Browse 为什么不再启动会话；完成标准是等待后 preview 仍能正常工作，并使用 branch 的 APP_URL。

由索引模型根据 Issue 内容生成。