feat: show page cart badge in blog post header (Phase 4)

Fetches page-scoped cart count from /internal/cart/summary?page_slug=
for page posts and displays a cart icon with count in the post header
that links to the page cart.

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

templates/_types/post/header/_header.html

@@ -12,7 +12,16 @@
         {{ post.title | truncate(160, True, '…') }}
       </span>
     {% endcall %}
-    {% call links.desktop_nav() %}    
+    {% call links.desktop_nav() %}
+      {% if page_cart_count is defined and page_cart_count > 0 %}
+        <a
+          href="{{ cart_url('/' + post.slug + '/') }}"
+          class="relative inline-flex items-center gap-1.5 px-3 py-1.5 text-sm rounded-full border border-emerald-300 bg-emerald-50 text-emerald-800 hover:bg-emerald-100 transition"
+        >
+          <i class="fa fa-shopping-cart" aria-hidden="true"></i>
+          <span>{{ page_cart_count }}</span>
+        </a>
+      {% endif %}
       {% include '_types/post/_nav.html' %}
     {% endcall %}
   {% endcall %}