UI idea: Multisig disaster recovery

The following scenario: After a disaster all that remains of the 2/3 multisignature wallet is:

1. seed 1
2. seed 2
3. xpub 3

There seems to be no intuitive way in Specter to recover. One has to know that one can create a custom device in Specter that contains only xpubs and is not from a signing device.

**Idea**
One could add a button (see below) to make clear one can add the public keys without a physical device:

Automatic conversions between different xpub types would also be extremely helpful. Something like: https://jlopp.github.io/xpub-converter/

Start with Specter's custom-device creation flow and the recovery scenario described in the issue, using the linked screenshot as the UI reference. Done should make adding public keys without a signing device discoverable and clarify whether automatic xpub-type conversion is included.

Written by the indexing model from the issue text.