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/live/shop/pages/product.ex

@@ -90,7 +90,7 @@ defmodule BerrypodWeb.Shop.Pages.Product do
   def handle_event("add_to_cart", _params, socket) do
     variant = socket.assigns.selected_variant
 
-    if variant do
+    if variant && variant.is_available do
       cart = Cart.add_item(socket.assigns.raw_cart, variant.id, socket.assigns.quantity)
 
       if socket.assigns[:analytics_visitor_hash] do