Restrict Rücksendung anfragen to delivered orders, fix Impressum email link

- customerOrderAction() only offers "Rücksendung anfragen" once an order
  is delivered, not already at shipped — a return before the package
  arrived doesn't make sense yet. UI-only change (a stricter subset of
  what the backend's CUSTOMER_ALLOWED_TRANSITIONS already permits).
- AnbieterAngaben.tsx's seller email is now a real mailto: link — it was
  plain text, the only non-clickable email on the site.

app/impressum/components/AnbieterAngaben.tsx

@@ -76,7 +76,9 @@ export function AnbieterAngaben({ seller }: { seller: CompanySettings }) {
           {seller.sellerZip} {seller.sellerCity}
         </P>
         <P>{seller.sellerCountry}</P>
-        <P>E-Mail: {seller.sellerEmail}</P>
+        <P>
+          E-Mail: <a href={`mailto:${seller.sellerEmail}`} className="text-brand hover:underline">{seller.sellerEmail}</a>
+        </P>
       </div>
 
       {seller.vatId && (