feat(user_live): handle system user in form and show

Early return / load_user_or_redirect, use system_user? to avoid editing system actor.

lib/mv_web/live/user_live/show.ex

@@ -75,7 +75,7 @@ defmodule MvWeb.UserLive.Show do
     actor = current_actor(socket)
     user = Ash.get!(Mv.Accounts.User, id, domain: Mv.Accounts, load: [:member], actor: actor)
 
-    if to_string(user.email) == Mv.Helpers.SystemActor.system_user_email() do
+    if Mv.Helpers.SystemActor.system_user?(user) do
       {:ok,
        socket
        |> put_flash(:error, gettext("This user cannot be viewed."))