extract setup wizard to dedicated /admin/setup page

Move the setup stepper out of the dashboard into its own LiveView.
Dashboard now redirects to setup when site isn't live, and shows
stats-only view once live. Also cleans up button component variant
handling, fixes alert CSS, and removes stale demo.html.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/berrypod_web/live/auth/confirmation_test.exs

@@ -64,7 +64,7 @@ defmodule BerrypodWeb.Auth.ConfirmationTest do
       assert Accounts.get_user!(user.id).confirmed_at
       # we are logged in now
       assert get_session(conn, :user_token)
-      assert redirected_to(conn) == ~p"/admin"
+      assert redirected_to(conn) == ~p"/admin/setup"
 
       # log out, new conn
       conn = build_conn()