Add back-in-stock notification signup form

NotifyMeForm.tsx replaces the disabled Add-to-cart button's spot once
a product/variant is out of stock — POSTs to the new
/api/stock-notifications route, which forwards to the backend's new
stock-notifications collection. Threaded product.numericId (not the
commerce slug id) into AddToCartButton/AddToCartInlineButton for this,
same split WishlistButton already uses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/cart/components/RelatedProducts.tsx

@@ -221,7 +221,7 @@ export function RelatedProducts({
                   lines (see ProductGrid.tsx's identical spacer). */}
               <div className="flex-1" />
 
-              <AddToCartInlineButton id={product.id} outOfStock={product.outOfStock} maxQty={product.maxQty} variants={product.variants} />
+              <AddToCartInlineButton id={product.id} numericId={product.numericId} outOfStock={product.outOfStock} maxQty={product.maxQty} variants={product.variants} />
             </div>
           </div>
           );