Use cart_url/page_cart_url for checkout action instead of url_for

The checkout form action used url_for('cart_global.checkout') which
only resolves inside the cart app. Replace with cart_url() and
page_cart_url() Jinja globals that work across all apps.

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

browser/templates/_types/cart/_cart.html

@@ -103,7 +103,7 @@
         {% if g.user %}
           <form
             method="post"
-            action="{{ url_for('page_cart.page_checkout')|host if page_post is defined and page_post else url_for('cart_global.checkout')|host }}"
+            action="{{ page_cart_url(page_post.slug, '/checkout/') if page_post is defined and page_post else cart_url('/checkout/') }}"
             class="w-full"
           >
             <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">