Sign in with an email link

[paccloud]

What to build

Add email-link sign-in next to Google (ADR 0001 in #122). A person types their email address, gets a link, opens it, and is signed in with the same data they would see after a Google sign-in on the same email. The password sign-in form is not offered in the new app.

Acceptance criteria

• The sign-in page offers "Continue with Google" and "Email me a sign-in link"
• Sending a link shows a clear "check your email" state
• Opening the link signs the person in, including when it is opened on a different device, where the app asks for the email address again
• An expired or reused link shows a plain error with a way to send a new link
• Tests cover the link flow against the Auth emulator
• No password sign-in form in the new app

Blocked by

• Custom yields in Firestore: Google sign-in and My data (tracer) #123