Group cart overview by market, show market name on cards

A page can have multiple markets — the overview now groups cart items
by market_place_id instead of page. Each card shows the market name
as heading with the page title below it.

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

templates/_types/cart/overview/_main_panel.html

@@ -33,7 +33,7 @@
           {% if grp.cart_items or grp.calendar_entries %}
 
             {% if grp.post %}
-              {# Page cart card #}
+              {# Market / page cart card #}
               <a
                 href="{{ cart_url('/' + grp.post.slug + '/') }}"
                 class="block rounded-2xl border border-stone-200 bg-white shadow-sm hover:shadow-md hover:border-stone-300 transition p-4 sm:p-5"
@@ -53,8 +53,15 @@
 
                   <div class="flex-1 min-w-0">
                     <h3 class="text-base sm:text-lg font-semibold text-stone-900 truncate">
-                      {{ grp.post.title }}
+                      {% if grp.market_place %}
+                        {{ grp.market_place.name }}
+                      {% else %}
+                        {{ grp.post.title }}
+                      {% endif %}
                     </h3>
+                    {% if grp.market_place %}
+                      <p class="text-xs text-stone-500 truncate">{{ grp.post.title }}</p>
+                    {% endif %}
 
                     <div class="mt-1 flex flex-wrap gap-2 text-xs text-stone-600">
                       {% if grp.product_count > 0 %}