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/cart/components/RelatedProducts.tsx

@@ -164,7 +164,7 @@ export function RelatedProducts() {
                 {product.name}
               </p>
               <p className="font-bold text-h4 text-text-primary">{formatPrice(product.price)}</p>
-              <AddToCartInlineButton id={product.id} variants={product.variants} />
+              <AddToCartInlineButton id={product.id} outOfStock={product.outOfStock} variants={product.variants} />
             </div>
           </div>
         ))}