Guided installer breaks when using an existing email address

[cerb]

Description

The guided installer fails / breaks the install process when the email address entered for the initial admin worker is already in use (e.g. team@cerb.ai).

Steps to reproduce

1. Start a fresh install and proceed through the guided installer.
2. On the admin worker setup step, enter an email address that already exists in the system (e.g. team@cerb.ai).
3. Submit the form.

Expected behavior

The installer should detect the conflict and surface a friendly validation error, allowing the user to choose a different address (or reuse the existing worker) without breaking the install flow.

Actual behavior

The install process breaks and cannot be completed.