feat: sync sessions via backend

[vslinko]

Description

I'd like to propose a feature to synchronize sessions via a custom server. I'm seeking feedback on whether this feature would be suitable for merging into the main repository. If the concept is well-received, I'll prepare a production-ready pull request that includes:

• Synchronization settings
• Configurable backend address
• Publishing a custom backend to GitHub and Docker Hub

Additional Notes

related to #1581 #1210 #702

Contributor Agreement

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• I agree to contribute all code submitted in this PR to the open-source community edition licensed under GPLv3 and the proprietary official edition without compensation.
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• I confirm that this code is my original work, or I have obtained the appropriate authorization from the copyright holder to submit this code under these terms.
• I understand that the submitted code will be publicly released under the GPLv3 license, and may also be used in the proprietary official edition.

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[chaoliu719]

That's really cool, I really need this feature. I run Chatbox on six different devices and often forget where to chat with GPT. I don't know why the official version doesn't have synchronization function. This is clearly not a very niche requirement, because chatbox has done a great job in multi terminal support. I am looking forward to the open-source version supporting synchronization.

[vslinko]

This pull request is no longer relevant for me. I've found a self-hosted alternative that better suits my needs. Feel free to merge this pull request at your discretion.