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/cart/components/CartContent.tsx

@@ -191,6 +191,19 @@ export function CartContent({
               const lowStock = entry.variant
                 ? (product.variants.find((v) => v.name === entry.variant)?.lowStock ?? false)
                 : product.lowStock;
+              // Same per-line resolution as lowStock above — caps how high
+              // the quantity stepper below can go, instead of only finding
+              // out at checkout that this many aren't actually available
+              // (api/checkout/route.ts's own stock check stays as the
+              // authoritative server-side guard). null (no cap) falls back
+              // to the stepper's original fixed 1-9 range; at least 1 is
+              // always offered even if maxQty is somehow lower than the
+              // qty already in this line, so the remove (×) button stays
+              // the only way down, never an empty <select>.
+              const maxQty = entry.variant
+                ? (product.variants.find((v) => v.name === entry.variant)?.maxQty ?? null)
+                : product.maxQty;
+              const qtyOptions = Array.from({ length: Math.max(1, Math.min(9, maxQty ?? 9)) }, (_, n) => n + 1);
               return (
               <div key={lineKey} className="w-full">
                 {i > 0 && <div className="h-px bg-border w-full mb-6" />}
@@ -238,7 +251,7 @@ export function CartContent({
                       onChange={(e) => setQuantity(product.id, Number(e.target.value), entry.variant)}
                       className="border border-border rounded-sm px-3.5 py-2 text-body-sm text-text-primary outline-none focus:border-brand transition-colors"
                     >
-                      {Array.from({ length: 9 }, (_, n) => n + 1).map((n) => (
+                      {qtyOptions.map((n) => (
                         <option key={n} value={n}>{n}</option>
                       ))}
                     </select>

app/cart/components/RelatedProducts.tsx

@@ -198,7 +198,7 @@ export function RelatedProducts({ defaultTaxRate }: { defaultTaxRate: number })
               <p className="min-h-[1.05rem] text-label font-bold text-warning">
                 {anyLowStock ? "Nur noch wenige verfügbar" : null}
               </p>
-              <AddToCartInlineButton id={product.id} outOfStock={product.outOfStock} variants={product.variants} />
+              <AddToCartInlineButton id={product.id} outOfStock={product.outOfStock} maxQty={product.maxQty} variants={product.variants} />
             </div>
           </div>
           );