[Bug] webdav in v2.9.7 will just overwrite the previous backup.json while login by another browser

**Describe the bug**
A clear and concise description of what the bug is.
webdav in v2.9.7 will just overwrite the previous backup.json while login by another browser

**To Reproduce**
Steps to reproduce the behavior:
1. sync your old conversation to cloud by webdav in A browser
2. login the same account and just sync your new conversation (or leave it blank) in B browser

**Expected behavior**
A clear and concise description of what you expected to happen.
old and new conversations should be merge.

**Screenshots**
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**Deployment**
- [ * ] Docker
- [ ] Vercel
- [ *] Server

**Desktop (please complete the following information):**
- OS: [e.g. iOS] win10
- Browser [e.g. chrome, safari] chrome,firefox
- Version [e.g. 22] v2.9.7

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional Logs**
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