This is basically done at time of writing. Including for sake of completeness for the parent EPIC, and to take the step to confirm that it actually works.
[ ] User can click login button in email and get taken to the website
[ ] User is immediately logged in and their browser local storage is populated with their private key
[ ] Assuming no other wallet attached the derived ethereum key is generated/confirmed and available to code that needs it
Summary
This is basically done at time of writing. Including for sake of completeness for the parent EPIC, and to take the step to confirm that it actually works.