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 }}

templates/fragments/nav_tree.html

@@ -0,0 +1,26 @@
+{# Nav-tree fragment — rendered by blog, consumed by all apps.
+   Uses frag_app_name / frag_first_seg instead of request.path / app_name
+   so the consuming app's context is reflected correctly.
+   No hx-boost — cross-app nav links are full page navigations. #}
+{% set _app_slugs = {'cart': cart_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">
+  {% from 'macros/scrolling_menu.html' import scrolling_menu with context %}
+  {% call(item) scrolling_menu('menu-items-container', menu_items) %}
+    {% set _href = _app_slugs.get(item.slug, blog_url('/' + item.slug + '/')) %}
+    <a
+      href="{{ _href }}"
+      aria-selected="{{ 'true' if (item.slug == frag_first_seg or item.slug == frag_app_name) else 'false' }}"
+      class="{{styles.nav_button_less_pad}}"
+    >
+      {% if item.feature_image %}
+        <img src="{{ item.feature_image }}"
+             alt="{{ item.label }}"
+             class="w-8 h-8 rounded-full object-cover flex-shrink-0" />
+      {% else %}
+        <div class="w-8 h-8 rounded-full bg-stone-200 flex-shrink-0"></div>
+      {% endif %}
+      <span>{{ item.label }}</span>
+    </a>
+  {% endcall %}
+</div>