Add admin nav link and reuse site header/footer on admin pages

Admins now see an Admin link in the main navigation, and the admin
section inherits the branded header/footer from layout/site.html.twig
instead of the bare base layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/layout/site.html.twig

@@ -21,6 +21,11 @@
                         <a class="nav-link" href="{{ path('game_dashboard') }}">{{ 'nav.game'|trans }}</a>
                     </li>
                     {% if app.user %}
+                        {% if is_granted('ROLE_ADMIN') %}
+                            <li class="nav-item">
+                                <a class="nav-link" href="{{ path('game_admin_dashboard') }}">{{ 'nav.admin'|trans }}</a>
+                            </li>
+                        {% endif %}
                         <li class="nav-item">
                             <a class="nav-link" href="{{ path('app_logout') }}">{{ 'nav.logout'|trans }}</a>
                         </li>