Fix cart template cross-app url_for crash and favicon 404

- Cart _cart.html: replace url_for('market.browse.product...') with
  market_product_url() for links and cart_global.update_quantity for
  quantity forms (market endpoints don't exist in cart app)
- Factory favicon route: use STATIC_DIR instead of relative "static"
  (resolves to shared/static/ where favicon.ico actually lives)
- Cart context processor: fetch all 3 fragments (cart-mini, auth-menu,
  nav-tree) concurrently, matching pattern in all other apps

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

cart/templates/_types/product/_cart.html

@@ -130,7 +130,7 @@
         class="w-24 h-24 sm:w-32 sm:h-28 rounded-xl border border-dashed border-stone-300 flex items-center justify-center text-xs text-stone-400"
       >
         No image
-      </div>'market', 'product', p.slug
+      </div>
     {% endif %}
   </div>
 
@@ -139,7 +139,7 @@
       <div class="flex flex-col sm:flex-row sm:items-start justify-between gap-2 sm:gap-3">
         <div class="min-w-0">
           <h2 class="text-sm sm:text-base md:text-lg font-semibold text-stone-900">
-            {% set href=url_for('market.browse.product.product_detail', product_slug=p.slug) %}
+            {% set href=market_product_url(p.slug) %}
             <a
               href="{{ href }}"
               hx_get="{{href}}"
@@ -188,12 +188,12 @@
       <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 = url_for('cart_global.update_quantity', product_id=p.id) %}
           <form
-            action="{{ url_for('market.browse.product.cart', product_slug=p.slug) }}"
+            action="{{ qty_url }}"
             method="post"
-            hx-post="{{ url_for('market.browse.product.cart', product_slug=p.slug) }}"
-            hx-target="#cart-mini"
-            hx-swap="outerHTML"
+            hx-post="{{ qty_url }}"
+            hx-swap="none"
           >
             <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
             <input
@@ -212,11 +212,10 @@
             {{ item.quantity }}
           </span>
           <form
-            action="{{ url_for('market.browse.product.cart', product_slug=p.slug) }}"
+            action="{{ qty_url }}"
             method="post"
-            hx-post="{{ url_for('market.browse.product.cart', product_slug=p.slug) }}"
-            hx-target="#cart-mini"
-            hx-swap="outerHTML"
+            hx-post="{{ qty_url }}"
+            hx-swap="none"
           >
             <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
             <input