Fix wishlist items not saving: pass customerId into toggleWishlistItem

The create POST omitted the customer relationship field entirely, so
added items never matched getWishlist's customer-scoped query. Also
scope the toggle-off fallback lookup to the current customer.

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

app/api/account/wishlist/route.ts

@@ -20,7 +20,7 @@ export async function POST(request: Request) {
     return NextResponse.json({ ok: false, reason: "Ungültiges Produkt." }, { status: 400 });
   }
 
-  const result = await toggleWishlistItem(session.token, productId, variant);
+  const result = await toggleWishlistItem(session.token, session.customer.id, productId, variant);
   if (!result.ok) return NextResponse.json({ ok: false, reason: "Merkliste konnte nicht aktualisiert werden." }, { status: 500 });
   return NextResponse.json({ ok: true, wishlisted: result.wishlisted });
 }