feat: do not require unlock password to recover encrypted scb

[rolznz]

This PR changes the how the Alby Account - specific static channel backups are encrypted to no longer require the unlock password, by deriving it from the seed. Therefore, the user only needs to backup their recovery phrase.

TODOs:

• change encryption (no need to re-generate key)
• should mnemonic exist for all backend types? not solving in this PR
• review derivation logic
• update derivation paths
  • It's very unlikely, but possible, that a given index does not produce a valid private key. Error checking is skipped in this example for brevity but should be handled in real code. In such a case, a ErrInvalidPrivateKey is returned. - what to do here if we use a hardcoded index for the backup? wait for the issue to happen (this will occur on node startup if ever, since we also store a channel backup on node startup)?
• review change to channel backup description
• update copy on backup page
• local E2E test