add provider sync enhancements for product lifecycle

- add discontinued status to products (soft-delete when removed from provider)
- add availability helpers to variants (available/out_of_stock/discontinued)
- add detailed sync audit logging (product created/updated/discontinued)
- add cost change detection with threshold alerts (5% warning, 20% critical)
- update cart to show unavailable items with appropriate messaging
- block checkout when cart contains unavailable items
- show discontinued badge on product pages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/theme-layer2-attributes.css

@@ -343,6 +343,11 @@
   color: #ffffff;
 }
 
+.badge-discontinued {
+  background-color: var(--t-text-tertiary, #737373);
+  color: #ffffff;
+}
+
 /* Product Card Images — mobile: swipe, desktop: hover crossfade */
 .product-card-image-wrap {
   position: relative;