Add cart + auth fragment pre-fetching to blog context

Phase 2 of fragment composition: blog_context() now concurrently
fetches cart-mini and auth-menu HTML fragments from cart and account
apps via fetch_fragments(). Updates shared submodule.

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

templates/_types/root/header/_header.html

@@ -0,0 +1,54 @@
+{% set select_colours = "
+  [.hover-capable_&]:hover:bg-yellow-300
+  aria-selected:bg-stone-500 aria-selected:text-white
+  [.hover-capable_&[aria-selected=true]:hover]:bg-orange-500
+"%}
+{% import 'macros/links.html' as links %}
+
+{% macro header_row(oob=False) %}
+  {% call links.menu_row(id='root-row', oob=oob) %}
+    <div class="w-full flex flex-row items-top">
+      {# Cart mini — fetched from cart app as fragment #}
+      {% if cart_mini_html %}
+        {{ cart_mini_html | safe }}
+      {% else %}
+        {% from '_types/cart/_mini.html' import mini with context %}
+        {{mini()}}
+      {% endif %}
+
+      {# Site title #}
+      <div class="font-bold text-5xl flex-1">
+        {% from 'macros/title.html' import title with context %}
+        {{ title('flex justify-center md:justify-start')}}
+      </div>
+
+      {# Desktop nav #}
+      <nav class="hidden md:flex gap-4 text-sm ml-2 justify-end items-center flex-0">
+        {% include '_types/root/_nav.html' %}
+        {# Auth menu — fetched from account app as fragment #}
+        {% if auth_menu_html %}
+          {{ auth_menu_html | safe }}
+        {% else %}
+          {% if not g.user %}
+            {% include '_types/root/_sign_in.html' %}
+          {% else %}
+            {% include '_types/root/_full_user.html' %}
+          {% endif %}
+        {% endif %}
+        {% include "_types/root/_nav_panel.html"%}
+      </nav>
+      {% include '_types/root/_hamburger.html' %}
+    </div>
+  {% endcall %}
+  {# Mobile user info #}
+  <div class="block md:hidden text-md font-bold">
+    {# Auth menu mobile — part of auth-menu fragment (id=auth-menu-mobile) #}
+    {% if not auth_menu_html %}
+      {% if g.user %}
+        {% include '_types/root/mobile/_full_user.html' %}
+      {% else %}
+        {% include '_types/root/mobile/_sign_in.html' %}
+      {% endif %}
+    {% endif %}
+  </div>
+{% endmacro %}