Cross-device synchronization

Currently, moving a 1wallet to a different device is tedious - the user has to use one of the three restoration methods to move their wallets, one by one. Each of them takes time. Since https://github.com/polymorpher/one-wallet/issues/190 will enable the user to provide an optional login (username/password), the user account can be also used for cross-device synchronization of non-transient wallet state and recovery information (e.g. the recovery Merkle tree). Some information can be stored under encryption using a 1wallet hosted server, or on user's Dropbox / Google Drive / iCloud Storage In doing so, the user would be able to maintain a consistent state and view of their wallets across many devices, and would conveniently and safely use the wallet anywhere.

Scheduled to be done in April 2022, released in v17.1

Started in https://github.com/polymorpher/one-wallet/pull/362, only recovery files are stored in the cloud for now. It would weaken the security if we store the wallet export files in server. So the next step would be to enable a server-assisted wallet synchronization session across devices, rather than storing anything in the server.