Authentication: sign up, sign in and sign out

[NomadicAlternative]

Source: specs/001-taskflow-mvp/spec.md — User Story 1 (P1)

Blocked on

The auth provider is still open in the spec (FR-011). Decide it in the meeting before starting.

Acceptance criteria

• A visitor can create an account with email and password
• An existing email is rejected with a visible validation message and no account is created
• A registered user can sign in and sign out
• Signed-out visitors who open a project URL are redirected to sign in instead of seeing content
• The signed-in user's identifier is available to the server-side data layer