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/versand/components/VersandSections.tsx

@@ -77,6 +77,10 @@ export function VersandSections({
           wir kostenlos.
         </p>
         <p>Alle angegebenen Preise verstehen sich inklusive der gesetzlichen Mehrwertsteuer.</p>
+        <p>
+          Rein digitale Produkte (z.B. Downloads) verursachen keine Versandkosten und sind von den
+          oben genannten Beträgen ausgenommen.
+        </p>
       </Section>
 
       <Section id="liefergebiet" title="Liefergebiet" withAnchor={withAnchors}>