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/checkout/components/CheckoutContent.tsx

@@ -7,7 +7,7 @@ import { useRouter } from "next/navigation";
 import { useCart, clearCart, mergeServerCartIntoLocal } from "../../lib/cart";
 import { useProducts } from "../../lib/products";
 import { useDiscount, clearDiscount } from "../../lib/discount";
-import { computeSubtotal, computeCartTotals, effectivePrice, effectiveTaxRate } from "../../lib/cartTotals";
+import { computeSubtotal, computeCartTotals, effectivePrice, effectiveTaxRate, cartHasShippableItem } from "../../lib/cartTotals";
 import { computeTaxBreakdown } from "@einfach-produktiv/invoicing";
 import { formatPrice } from "../../lib/format";
 import { Reveal } from "../../components/Reveal";
@@ -372,7 +372,7 @@ export function CheckoutContent({
     selectedShipping?.freeShippingThreshold !== null &&
     selectedShipping?.freeShippingThreshold !== undefined &&
     subtotal >= selectedShipping.freeShippingThreshold;
-  const shipping = items.length === 0 || freeShipping ? 0 : selectedShipping?.price ?? 0;
+  const shipping = items.length === 0 || !cartHasShippableItem(items) || freeShipping ? 0 : selectedShipping?.price ?? 0;
   const { totalSavings, discountAmount, total } = computeCartTotals(items, shipping, discount);
   const taxBreakdown = computeTaxBreakdown(
     items.map(({ entry, product }) => ({
@@ -1337,33 +1337,39 @@ export function CheckoutContent({
               </div>
             )}
 
-            <div className="flex flex-col gap-0.5 w-full">
-              <div className="flex items-center w-full">
-                <span className="flex items-center gap-1.5 text-body-sm text-text-primary">
-                  Versand
-                  <button
-                    type="button"
-                    onClick={() => setVersandOpen(true)}
-                    className="text-text-muted hover:text-brand transition-colors"
-                    aria-label="Alle Informationen zu Versandkosten und Lieferzeiten"
-                  >
-                    <span aria-hidden>ⓘ</span>
-                  </button>
-                </span>
-                <span className="flex-1" />
-                <span className="text-body-sm text-text-primary">
-                  {displayShipping === 0 ? "Kostenlos" : formatPrice(displayShipping)}
-                </span>
+            {/* Hidden entirely (not just "Kostenlos") when nothing in the
+                cart actually triggers shipping at all — a different state
+                from hitting the free-shipping threshold, which is still a
+                real promotional message worth showing. */}
+            {cartHasShippableItem(items) && (
+              <div className="flex flex-col gap-0.5 w-full">
+                <div className="flex items-center w-full">
+                  <span className="flex items-center gap-1.5 text-body-sm text-text-primary">
+                    Versand
+                    <button
+                      type="button"
+                      onClick={() => setVersandOpen(true)}
+                      className="text-text-muted hover:text-brand transition-colors"
+                      aria-label="Alle Informationen zu Versandkosten und Lieferzeiten"
+                    >
+                      <span aria-hidden>ⓘ</span>
+                    </button>
+                  </span>
+                  <span className="flex-1" />
+                  <span className="text-body-sm text-text-primary">
+                    {displayShipping === 0 ? "Kostenlos" : formatPrice(displayShipping)}
+                  </span>
+                </div>
+                <p className="text-label text-text-muted">
+                  {shipping === 0 && selectedShipping?.freeShippingThreshold != null
+                    ? `ab ${formatPrice(selectedShipping.freeShippingThreshold)} innerhalb Deutschlands`
+                    : (selectedShipping?.description ?? "innerhalb Deutschlands")}
+                </p>
+                <p className="text-label text-text-muted">
+                  Lieferzeit {shippingSettings.totalDays.min}–{shippingSettings.totalDays.max} Werktage
+                </p>
               </div>
-              <p className="text-label text-text-muted">
-                {shipping === 0 && selectedShipping?.freeShippingThreshold != null
-                  ? `ab ${formatPrice(selectedShipping.freeShippingThreshold)} innerhalb Deutschlands`
-                  : (selectedShipping?.description ?? "innerhalb Deutschlands")}
-              </p>
-              <p className="text-label text-text-muted">
-                Lieferzeit {shippingSettings.totalDays.min}–{shippingSettings.totalDays.max} Werktage
-              </p>
-            </div>
+            )}
 
             <div className="h-px bg-border w-full" />