Add real order persistence, customer accounts, and cart sync

Checkout now persists orders server-side (Payload orders collection,
re-priced from live product data, discount codes redeemed exactly once)
instead of writing a client-only sessionStorage snapshot. Buying requires
an account (registration inline in checkout, no separate step) — accounts
get order history with delivery status, profile/address editing, password
change, and a cart that syncs across devices while logged in.

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

app/bestellbestaetigung/components/BestellbestaetigungContent.tsx

@@ -263,6 +263,12 @@ export function BestellbestaetigungContent() {
         </Reveal>
       )}
 
+      <Reveal delay={0.08} className="flex items-center justify-center pb-4 px-[var(--layout-padding-x)] w-full">
+        <Link href="/konto/bestellungen" className="underline text-body-sm text-text-primary hover:text-brand transition-colors">
+          Meine Bestellungen ansehen
+        </Link>
+      </Reveal>
+
       {/* Testimonial band — same proven structure as /not-found's version
           (see that page's own comment), not a new layout: w-[45%] image
           column, narrow 40px edge gradient into bg-muted, quote in the