http://localhost:3000. You sign in with a developer account.
Developer accounts
The first developer account is created automatically on first startup from theADMIN_EMAIL and ADMIN_PASSWORD environment variables. The seed runs only while no developer account exists, so change the default password from Settings → Change Password right after your first login.
Every further account is created by invitation from Settings → Team. There is no self-registration. All developer accounts have the same permissions, see Team.

