Cap add-to-cart quantity at actual remaining stock

Stock was only checked at checkout; a shopper could add more of a
product to the cart than was actually in stock and only find out at
the last step. Product/variant now carry a real maxQty, and
AddToCartButton/AddToCartInlineButton/the cart's quantity stepper all
disable or cap once the cart already holds that many.

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

app/todo-cards/components/Pricing.tsx

@@ -103,6 +103,7 @@ export async function Pricing() {
             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"
             outOfStock={product.outOfStock}
+            maxQty={product.maxQty}
             variants={product.variants}
           />
         </div>

app/todo-cards/components/TodoKartenHero.tsx

@@ -119,7 +119,7 @@ export async function TodoKartenHero() {
             </div>
 
             {product && (
-              <AddToCartButton label="ToDo-Karten bestellen" outOfStock={product.outOfStock} variants={product.variants} />
+              <AddToCartButton label="ToDo-Karten bestellen" outOfStock={product.outOfStock} maxQty={product.maxQty} variants={product.variants} />
             )}
           </div>
         </Reveal>