feat: add a page to recover when an account turns out to be already used while adding it

This page is a bit of a bodge for the fact that the current Accounts model must be 'unlocked' before it can be used.
In turn, this means that it is not straightforward to check that an account is still 'available' when presenting the user with the option to add an account received via URI from the commandline.
The solution implemented here is to check and let the user recover after unlocking, if necessary.

Issues: #7, #14