Add nav-tree fragment: blog renders nav for all apps

Blog now provides a nav-tree fragment at /internal/fragments/nav-tree
that accepts app_name and path params for correct aria-selected
highlighting. Blog itself consumes this fragment alongside cart-mini
and auth-menu in a single concurrent fetch_fragments() call.

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

templates/_types/root/header/_header.html

@@ -24,7 +24,11 @@
 
       {# Desktop nav #}
       <nav class="hidden md:flex gap-4 text-sm ml-2 justify-end items-center flex-0">
-        {% include '_types/root/_nav.html' %}
+        {% if nav_tree_html %}
+          {{ nav_tree_html | safe }}
+        {% else %}
+          {% include '_types/root/_nav.html' %}
+        {% endif %}
         {# Auth menu — fetched from account app as fragment #}
         {% if auth_menu_html %}
           {{ auth_menu_html | safe }}