Fix gutter width: close header wrapper before dark main area

The max-w-screen-2xl wrapper now only constrains the header/nav,
matching blog layout. Dark content area goes full-width with its
own inner max-w constraint.

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

common/artdag_common/templates/_base.html

@@ -82,6 +82,8 @@
     {# App-specific sub-nav (Runs, Recipes, Effects, etc.) #}
     {% block sub_nav %}{% endblock %}
 
+    </div>{# close max-w-screen-2xl wrapper #}
+
     <main class="bg-dark-800 text-gray-100 min-h-screen">
       <div class="max-w-screen-2xl mx-auto px-4 py-4">
         {% block content %}{% endblock %}
@@ -89,7 +91,6 @@
     </main>
 
     {% block footer %}{% endblock %}
-    </div>
     {% block scripts %}{% endblock %}
 </body>
 </html>