Out-of-stock UI, variant picker on marketing pages, server-side stock check

- ProductGrid/AddToCartInlineButton/AddToCartButton now show "Ausverkauft"
  and disable add-to-cart per variant (or product-level with no variants),
  derived from trackInventory/stock/allowBackorder via isOutOfStock().
- AddToCartButton (todo-cards Hero+Pricing, homepage spotlight) gains the
  same variant <select> AddToCartInlineButton already had — all three call
  sites already fetch full product data server-side.
- /api/checkout re-validates stock server-side (depth-in-defense, not just
  the disabled button), rejecting when trackInventory is on, allowBackorder
  is off, and requested qty exceeds stock.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018PL4zfTY1sXc8x5QS6FatM

app/todo-cards/components/Pricing.tsx

@@ -79,6 +79,8 @@ 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"
+            outOfStock={product.outOfStock}
+            variants={product.variants}
           />
         </div>
       </Reveal>

app/todo-cards/components/TodoKartenHero.tsx

@@ -107,7 +107,9 @@ export async function TodoKartenHero() {
               </p>
             </div>
 
-            <AddToCartButton label="ToDo-Karten bestellen" />
+            {product && (
+              <AddToCartButton label="ToDo-Karten bestellen" outOfStock={product.outOfStock} variants={product.variants} />
+            )}
           </div>
         </Reveal>