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>

lib/berrypod_web/components/shop_components/cart.ex

@@ -212,6 +212,10 @@ defmodule BerrypodWeb.ShopComponents.Cart do
           </p>
         <% end %>
 
+        <p :if={Map.get(@item, :is_available) == false} class="cart-item-unavailable">
+          This item is currently unavailable
+        </p>
+
         <div class="cart-item-actions">
           <%= if @show_quantity_controls do %>
             <form