fix(auth): clarify login identifier field

[lntutor]

Summary by cubic

Clarified the login identifier by changing the field from “Email address” to “Username or email” and submitting it as username. This removes confusion for users who sign in with a username.

• Bug Fixes
  • Updated i18n: label, placeholder, and required message to “Username or email”.
  • Renamed form field email → username; added autocomplete="username"; validation errors now show on submit.
  • Password login now posts username to the API.
  • Tests: added unit tests; updated Cypress and data layer selectors to username.

^{Written for commit b656b28. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[dokterbob]

Interesting contribution. Just, I am wondering whether it'll actually reduce confusion, overall: some users will user usernames, others will use emails, some will use both.

Perhaps a more 'neutral' way is to just talk about 'Login', which is close enough to identifier that developers will understand it and close enough to username that users might.

Let me know how you feel about this. Appreciate the added frontend unit test, for sure!