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/page_renderer.ex

@@ -553,7 +553,18 @@ defmodule BerrypodWeb.PageRenderer do
             </div>
 
             <.quantity_selector quantity={assigns[:quantity] || 1} in_stock={@product.in_stock} />
-            <.add_to_cart_button mode={@mode} />
+
+            <p
+              :if={assigns[:selected_variant] && !assigns[:selected_variant].is_available}
+              class="variant-unavailable-msg"
+            >
+              This option is currently unavailable
+            </p>
+
+            <.add_to_cart_button
+              mode={@mode}
+              disabled={assigns[:selected_variant] && !assigns[:selected_variant].is_available}
+            />
           </form>
         </div>
       </div>