PasstDazu driven by Products.relatedProduct; circle back on Focus icons

- PasstDazu.tsx now takes a productSlug prop and reads its cross-sell
  target from the product's own relatedProduct field (Payload) instead
  of hardcoding "todo-karten" — reusable on any product page now.
- payload.ts: Product/PayloadProduct gained relatedProduct, resolved via
  a bounded-recursion mapPayloadProduct call (safe — the fetch's depth:2
  means the nested relation's own relatedProduct is never populated).
- Focus.tsx icons: added the black stroked circle back around each glyph
  (matches icon-step-3.png exactly) — dropped in the previous pass by
  mistake while fixing the color/fill.

app/der-eine/page.tsx

@@ -44,7 +44,7 @@ export default async function DerEinePage() {
       <main className="flex flex-col flex-1">
         <Hero />
         <Focus />
-        <PasstDazu />
+        <PasstDazu productSlug="stift-kugelschreiber" />
       </main>
       <Footer />
     </>