Download and import chat history backup

An option to download the chat history backup locally, and to import such a backup file. Backups stored on Venice servers can only be restored to the same account, this should allow importing to a different account.

There are several reasons:

1. To preserve chat history when merging separate accounts (e.g. email + web3 account).

2. For people who don’t want to have their backup stored on Venice servers (despite the encryption).

There is a related request, but I think this is a separate feature:

• https://veniceai.featurebase.app/p/batch-export-chats For using the data outside the web app. If it could also be imported into the web app, then it would serve the same purpose.