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/todo-cards/components/Pricing.tsx

@@ -125,6 +125,7 @@ export async function Pricing() {
           <AddToCartButton
             label="In den Warenkorb"
             className="w-full inline-flex items-center justify-center px-6 py-[0.8125rem] rounded-sm bg-brand hover:bg-brand-hover active:scale-[0.97] transition-all font-bold text-body text-text-primary text-center focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand focus-visible:ring-offset-2 focus-visible:ring-offset-bg-muted"
+            numericId={product.numericId}
             outOfStock={!product.active || product.outOfStock}
             maxQty={product.maxQty}
             variants={product.active ? product.variants : []}

app/todo-cards/components/TodoKartenHero.tsx

@@ -149,6 +149,7 @@ export async function TodoKartenHero() {
               // isn't enough once variants is non-empty.
               <AddToCartButton
                 label="ToDo-Karten bestellen"
+                numericId={product.numericId}
                 outOfStock={!product.active || product.outOfStock}
                 maxQty={product.maxQty}
                 variants={product.active ? product.variants : []}