Bank accounts should be visible in the involved parties field

When booking bank transfers we need to specify between which bank accounts the money is transferred. I suggest adding the bank accounts as a source field in the 'members' dropdown.

Start by locating the bank-transfer booking flow and its “members” dropdown. Trace how the dropdown's source options are assembled, then verify that bank accounts can be selected as transfer parties and that the resulting booking records the intended accounts.

Written by the indexing model from the issue text.