Add Art-DAG to coop nav-tree fragment

Append synthetic artdag nav entry to blog's nav-tree handler so
Art-DAG appears in the shared navigation across all 6 coop apps.
Register artdag_url as Jinja global.

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

blog/templates/fragments/nav_tree.html

@@ -8,6 +8,7 @@
   'events': events_url('/'),
   'federation': federation_url('/'),
   'account': account_url('/'),
+  'artdag': artdag_url('/'),
 } %}
 <div class="flex flex-col sm:flex-row sm:items-center gap-2 border-r border-stone-200 mr-2 sm:max-w-2xl"
      id="menu-items-nav-wrapper">