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>

lib/simpleshop_theme_web/live/admin/theme/index.html.heex

@@ -35,7 +35,14 @@
           </button>
         </div>
       <% else %>
-        <!-- Header -->
+        <.link
+          href={~p"/admin/orders"}
+          class="inline-flex items-center gap-1 text-sm text-base-content/60 hover:text-base-content mb-4"
+        >
+          <.icon name="hero-arrow-left-mini" class="size-4" /> Admin
+        </.link>
+        
+<!-- Header -->
         <div class="mb-6 flex items-start justify-between gap-3">
           <div class="flex-1">
             <h1 class="text-xl font-semibold tracking-tight mb-2 text-base-content">