Render private key fingerprint for backups

[martgil]

When rendering the FlowCrypt user backup in an email, there's no way a user can identify its fingerprint not unless a user disables FlowCrypt browser extension through the browser's settings and look the for the fingerprint that way.

I'll be extremely helpful if the key's fingerprint is visible for each backup whether or not its already imported. So that removing unnecessary backup will be much easier to do since user can already see the key's fingerprint displayed right away.

[martgil]

This will be useful for this documentation guide here once implemented: https://github.com/FlowCrypt/flowcrypt-web/pull/1997