add admin sidebar layout with responsive drawer navigation

- New admin root + child layouts with daisyUI drawer sidebar
- AdminLayoutHook tracks current path for active nav highlighting
- Split router into :admin, :admin_theme, :user_settings live_sessions
- Theme editor stays full-screen with back link to admin
- Admin bar on shop pages for logged-in users (mount_current_scope)
- Strip Layouts.app wrapper from admin LiveViews
- Remove nav from root.html.heex (now only serves auth pages)
- 9 new layout tests covering sidebar, active state, theme editor, admin bar

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

test/simpleshop_theme_web/live/auth/login_test.exs

@@ -9,7 +9,7 @@ defmodule SimpleshopThemeWeb.Auth.LoginTest do
       {:ok, _lv, html} = live(conn, ~p"/users/log-in")
 
       assert html =~ "Log in"
-      assert html =~ "Register"
+      assert html =~ "Sign up"
       assert html =~ "Log in with email"
     end
   end