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/cart/components/CartContent.tsx

@@ -7,7 +7,7 @@ import Image from "next/image";
 import { useCart, removeFromCart, setQuantity } from "../../lib/cart";
 import { useProducts } from "../../lib/products";
 import { useDiscount, applyDiscount, 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, discountPercent } from "../../lib/format";
 import { Reveal } from "../../components/Reveal";
@@ -77,7 +77,7 @@ export function CartContent({
 
   const subtotal = computeSubtotal(items);
   const shipping =
-    items.length === 0 || (freeShippingThreshold !== null && subtotal >= freeShippingThreshold)
+    items.length === 0 || !cartHasShippableItem(items) || (freeShippingThreshold !== null && subtotal >= freeShippingThreshold)
       ? 0
       : shippingCost;
   const { totalSavings, discountAmount, total } = computeCartTotals(items, shipping, discount);
@@ -392,31 +392,40 @@ export function CartContent({
               )}
 
               <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">
-                    {shipping === 0 ? "Kostenlos" : formatPrice(shipping)}
-                  </span>
-                </div>
-                <p className="text-label text-text-muted">
-                  {shipping === 0 && freeShippingThreshold !== null
-                    ? `ab ${formatPrice(freeShippingThreshold)} innerhalb Deutschlands`
-                    : "innerhalb Deutschlands"}
-                </p>
-                <p className="text-label text-text-muted">
-                  Lieferzeit {shippingSettings.totalDays.min}–{shippingSettings.totalDays.max} Werktage
-                </p>
+                {/* Hidden entirely (not just "Kostenlos") when nothing in
+                    the cart actually triggers shipping at all — that's a
+                    different state from hitting the free-shipping
+                    threshold, which is still a real promotional message
+                    worth showing. */}
+                {cartHasShippableItem(items) && (
+                  <>
+                    <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">
+                        {shipping === 0 ? "Kostenlos" : formatPrice(shipping)}
+                      </span>
+                    </div>
+                    <p className="text-label text-text-muted">
+                      {shipping === 0 && freeShippingThreshold !== null
+                        ? `ab ${formatPrice(freeShippingThreshold)} innerhalb Deutschlands`
+                        : "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" />