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/controllers/user_session_controller_test.exs

@@ -20,12 +20,10 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert get_session(conn, :user_token)
       assert redirected_to(conn) == ~p"/"
 
-      # Now do a logged in request to an admin page and assert on the menu
+      # Now do a logged in request and assert on the page content
       conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
-      assert response =~ ~p"/users/settings"
-      assert response =~ ~p"/users/log-out"
     end
 
     test "logs the user in with remember me", %{conn: conn, user: user} do
@@ -84,12 +82,10 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
       assert get_session(conn, :user_token)
       assert redirected_to(conn) == ~p"/"
 
-      # Now do a logged in request to an admin page and assert on the menu
+      # Now do a logged in request and assert on the page content
       conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
-      assert response =~ ~p"/users/settings"
-      assert response =~ ~p"/users/log-out"
     end
 
     test "confirms unconfirmed user", %{conn: conn, unconfirmed_user: user} do
@@ -108,12 +104,10 @@ defmodule SimpleshopThemeWeb.UserSessionControllerTest do
 
       assert Accounts.get_user!(user.id).confirmed_at
 
-      # Now do a logged in request to an admin page and assert on the menu
+      # Now do a logged in request and assert on the page content
       conn = get(conn, ~p"/users/settings")
       response = html_response(conn, 200)
       assert response =~ user.email
-      assert response =~ ~p"/users/settings"
-      assert response =~ ~p"/users/log-out"
     end
 
     test "redirects to login page when magic link is invalid", %{conn: conn} do