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/lib/__tests__/cartTotals.test.ts

@@ -27,6 +27,7 @@ const product = (overrides: Partial<Product> = {}): Product => ({
   taxRatePercent: null,
   noShippingCost: false,
   categories: [],
+  relatedProduct: null,
   ...overrides,
 });
 

app/lib/payload.ts

@@ -251,6 +251,12 @@ export type Product = {
   // than "matches none", since an uncategorized product shouldn't just
   // disappear the moment a category filter is applied.
   categories: string[];
+  // Optional cross-sell pick (Products.relatedProduct) — powers a "Passt
+  // dazu" card on the product's own detail page, same field/pattern as
+  // Posts.relatedProduct. null means no card. Only one level deep (this
+  // related product's own relatedProduct is never resolved/shown) — see
+  // mapPayloadProduct's own comment.
+  relatedProduct: Product | null;
 };
 
 type PayloadProduct = {
@@ -288,6 +294,7 @@ type PayloadProduct = {
       }[]
     | null;
   categories: ({ name: string } | number)[] | null;
+  relatedProduct: PayloadProduct | number | null;
 };
 
 // A product/variant is only actually unbuyable when it opted into
@@ -354,6 +361,10 @@ export function mapPayloadProduct(product: PayloadProduct): Product {
     categories: (product.categories ?? [])
       .map((c) => (typeof c === "object" && c ? c.name : null))
       .filter((name): name is string => Boolean(name)),
+    // Recursive call is safe here — depth=2 on the originating fetch means
+    // this nested object's OWN relatedProduct is never populated (stays a
+    // raw id or absent), so the recursion bottoms out after exactly one level.
+    relatedProduct: typeof product.relatedProduct === "object" && product.relatedProduct ? mapPayloadProduct(product.relatedProduct) : null,
   };
 }