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/lib/payload.ts

@@ -170,7 +170,13 @@ export type Product = {
   spotlightHeadline: string | null;
   spotlightText: string | null;
   spotlightImage: string | null;
-  variants: { name: string; priceOverride: number | null }[];
+  // Plain booleans, not the raw stock/threshold numbers — the public API
+  // has no reason to leak exact stock counts, callers only ever need
+  // "can this be bought right now". `outOfStock` on the product itself
+  // only matters for a product with no variants; a varianted product's
+  // buyability is entirely per-variant (see each variant's own flag).
+  outOfStock: boolean;
+  variants: { name: string; priceOverride: number | null; outOfStock: boolean }[];
 };
 
 type PayloadProduct = {
@@ -189,9 +195,21 @@ type PayloadProduct = {
   spotlightHeadline: string | null;
   spotlightText: string | null;
   spotlightImage: { url: string } | number | null;
-  variants: { name: string; priceOverride: number | null }[] | null;
+  trackInventory: boolean;
+  stock: number | null;
+  allowBackorder: boolean;
+  variants: { name: string; priceOverride: number | null; trackInventory: boolean; stock: number | null; allowBackorder: boolean }[] | null;
 };
 
+// A product/variant is only actually unbuyable when it opted into
+// inventory tracking AND has zero stock AND backorders aren't allowed —
+// the same three-condition check lib/inventory.ts's adjustStock() effectively
+// mirrors from the other direction (it only ever touches stock when
+// trackInventory is on in the first place).
+function isOutOfStock(trackInventory: boolean, stock: number | null, allowBackorder: boolean): boolean {
+  return trackInventory && !allowBackorder && (stock ?? 0) <= 0;
+}
+
 // Shared by getProducts() and getPostBySlug()'s relatedProduct — kept in
 // one place instead of duplicating the same field mapping, which is
 // exactly the kind of drift this session's Shipping Settings work was
@@ -213,7 +231,12 @@ export function mapPayloadProduct(product: PayloadProduct): Product {
     spotlightText: product.spotlightText || null,
     spotlightImage:
       typeof product.spotlightImage === "object" && product.spotlightImage ? product.spotlightImage.url : null,
-    variants: product.variants ?? [],
+    outOfStock: isOutOfStock(product.trackInventory, product.stock, product.allowBackorder),
+    variants: (product.variants ?? []).map((v) => ({
+      name: v.name,
+      priceOverride: v.priceOverride,
+      outOfStock: isOutOfStock(v.trackInventory, v.stock, v.allowBackorder),
+    })),
   };
 }