Sign-in labels the single identity field 'Username', but `login()` sends it as
`email` and dcntrlized-auth resolves accounts by the email column. Typing the
username of a seeded account ('admin') therefore fails with 'invalid
credentials' while the password is correct. The field is now labelled Email,
with an email keyboard and a matching placeholder, in sign-in mode only —
registration still has a separate username and email.
Admin
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