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/user_auth_test.exs

@@ -25,7 +25,7 @@ defmodule BerrypodWeb.UserAuthTest do
       conn = UserAuth.log_in_user(conn, user)
       assert token = get_session(conn, :user_token)
       assert get_session(conn, :live_socket_id) == "users_sessions:#{Base.url_encode64(token)}"
-      assert redirected_to(conn) == ~p"/admin"
+      assert redirected_to(conn) == ~p"/admin/setup"
       assert Accounts.get_user_by_session_token(token)
     end
 
@@ -80,7 +80,7 @@ defmodule BerrypodWeb.UserAuthTest do
         |> assign(:current_scope, Scope.for_user(user))
         |> UserAuth.log_in_user(user)
 
-      assert redirected_to(conn) == ~p"/admin"
+      assert redirected_to(conn) == ~p"/admin/setup"
     end
 
     test "writes a cookie if remember_me was set in previous session", %{conn: conn, user: user} do