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/konto/merkliste/components/MerklisteGrid.tsx

@@ -105,7 +105,7 @@ export function MerklisteGrid({
                   not a merge), so passing just "w-full" here previously threw
                   away all the button's actual styling. Its default is
                   already `w-full`. */}
-              <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>
           </RevealItem>
         );