prevent checkout with unavailable product variants

- Include is_available flag in hydrated cart items
- Show unavailable message on cart items and product page
- Block add-to-cart for unavailable variants
- Redirect back to cart with error if checkout has unavailable items

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

assets/css/shop/components.css

@@ -781,6 +781,15 @@
     font-weight: 600;
   }
 
+  /* ── Unavailable message ── */
+
+  .variant-unavailable-msg {
+    color: var(--t-sale-color);
+    font-size: var(--t-text-small);
+    font-weight: 500;
+    margin-bottom: 1rem;
+  }
+
   /* ── Add to cart ── */
 
   .atc-wrap {
@@ -1709,6 +1718,13 @@
     margin: 0;
   }
 
+  .cart-item-unavailable {
+    font-size: var(--t-text-caption);
+    color: var(--t-sale-color);
+    font-weight: 500;
+    margin: 0.25rem 0 0;
+  }
+
   .cart-item-actions {
     display: flex;
     justify-content: space-between;