Index: filter out SystemActor.system_user_email() in query. Show/Form:
redirect to /users with flash when viewing or editing system actor user.
Index format_error: handle Ash errors without :message field.
- Replace Ash.get!/Ash.destroy! with Ash.get/Ash.destroy
- Add case statements for Forbidden, NotFound, and generic errors
- Display user-friendly flash messages for all error cases
- Use Enum.map_join/3 for efficient error formatting
