Fix cross-origin cart +/- buttons by supporting cart_quantity_url in template

The cart_item macro now checks for cart_quantity_url (defined only in the cart app)
and uses it for same-origin quantity updates, falling back to market_product_url.

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

browser/templates/_types/product/_cart.html

@@ -188,10 +188,11 @@
       <div class="mt-3 flex flex-col sm:flex-row sm:items-center justify-between gap-2 sm:gap-4">
         <div class="flex items-center gap-2 text-xs sm:text-sm text-stone-700">
           <span class="text-[0.65rem] sm:text-xs uppercase tracking-wide text-stone-500">Quantity</span>
+          {% set qty_url = cart_quantity_url(item.product_id) if cart_quantity_url is defined else market_product_url(p.slug, 'cart', item.market_place) %}
           <form
-            action="{{ market_product_url(p.slug, 'cart', item.market_place) }}"
+            action="{{ qty_url }}"
             method="post"
-            hx-post="{{ market_product_url(p.slug, 'cart', item.market_place) }}"
+            hx-post="{{ qty_url }}"
             hx-target="#cart-mini"
             hx-swap="outerHTML"
           >
@@ -212,9 +213,9 @@
             {{ item.quantity }}
           </span>
           <form
-            action="{{ market_product_url(p.slug, 'cart', item.market_place) }}"
+            action="{{ qty_url }}"
             method="post"
-            hx-post="{{ market_product_url(p.slug, 'cart', item.market_place) }}"
+            hx-post="{{ qty_url }}"
             hx-target="#cart-mini"
             hx-swap="outerHTML"
           >