Personal auto-accounts use usernames of the form goa-bank-acc- plus a random
hex id so they are easy to spot in admin lists and distinct from explorer
or hand-registered users.
Add GET /intro/auto-account.json on the bank helper (demo-withdraw-api):
public registration with generated username/password, balance starts GOA:0,
credentials returned once for the client to display.
Nginx proxies the new path; install-demo-withdraw-api.sh installs the
location. This is separate from the shared explorer demo withdraw.
