Honor Products.noShippingCost across cart, checkout, and product pages

- api/checkout/route.ts: authoritative shipping charge is 0 whenever
  every cart line opts out via noShippingCost, regardless of the
  free-shipping threshold.
- Cart/checkout order summaries: the whole "Versand" line (cost, free-
  shipping note, delivery time) is hidden entirely rather than showing
  "Kostenlos" — that's a different state from hitting the threshold.
- ProductSpotlight/Pricing/TodoKartenHero: "zzgl. Versand" and delivery-
  time hints drop for an exempted product's own page.
- /versand + its shared modal: one clarifying sentence that digital
  products are exempt.
- Widerrufsformular link: opens inline in a new tab (no forced download),
  arrow icon changed from a download glyph to a plain right arrow to
  match.

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

app/api/checkout/route.ts

@@ -198,12 +198,19 @@ export async function POST(request: Request) {
     });
   }
   const subtotal = roundMoney(items.reduce((sum, i) => sum + i.quantity * i.unitPrice, 0));
+  // Products.noShippingCost — a cart made up entirely of items that opt
+  // out of shipping costs (e.g. purely digital downloads) never gets
+  // charged shipping at all, regardless of the free-shipping threshold.
+  // A single item WITHOUT the flag still triggers normal shipping for the
+  // whole order — this only exempts a product, never the whole cart just
+  // because it contains an exempt item.
+  const hasShippableItem = body.cart.some((line) => !productsBySlug.get(line.id)?.noShippingCost);
 
   const shippingMethods = await getShippingMethods();
   const shippingMethod = shippingMethods.find((m) => m.id === body.shippingMethodId);
   if (!shippingMethod) return NextResponse.json({ ok: false, reason: "Versandart ist ungültig." }, { status: 400 });
   const freeShipping = shippingMethod.freeShippingThreshold != null && subtotal >= shippingMethod.freeShippingThreshold;
-  const shippingCost = freeShipping ? 0 : shippingMethod.price;
+  const shippingCost = !hasShippableItem || freeShipping ? 0 : shippingMethod.price;
 
   const paymentMethods = await getPaymentMethods();
   const paymentMethod = paymentMethods.find((m) => m.id === body.paymentMethodId);