Blog categories (hasMany), shipping-address contact fields, product SKU on invoices

- Posts.categories is now hasMany — blog list/detail/live-preview render
  a comma-joined list instead of a single category.
- Checkout's "Abweichende Lieferadresse" gains optional Firma + Kontakt-
  E-Mail/Telefon fields (handed to the shipping carrier, not used for
  customer communication).
- Customer profile can now store its own shipping address (mirroring
  Customers.ts's new "Lieferadresse" tab), prefilling the checkout
  override instead of always starting blank.
- Product-level optional SKU (previously only on variants) snapshots onto
  each order item and shows up on invoices (visual + EN16931 XML),
  the confirmation email, and the account order detail page.

app/lib/payload.ts

@@ -21,7 +21,7 @@ export type BlogPost = {
   id: number;
   title: string;
   slug: string;
-  category: string;
+  categories: string[];
   readTime: number;
   excerpt: string;
   thumbnail: string | null;
@@ -33,7 +33,7 @@ type PayloadPost = {
   id: number;
   title: string;
   slug: string;
-  category: { name: string } | number | null;
+  categories: ({ name: string } | number)[];
   readTime: number;
   excerpt: string;
   thumbnail: { url: string } | number | null;
@@ -73,10 +73,9 @@ export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
     id: post.id,
     title: post.title,
     slug: post.slug,
-    category:
-      typeof post.category === "object" && post.category
-        ? post.category.name
-        : "",
+    categories: (post.categories ?? [])
+      .map((c) => (typeof c === "object" && c ? c.name : null))
+      .filter((name): name is string => Boolean(name)),
     readTime: post.readTime,
     excerpt: post.excerpt,
     thumbnail:
@@ -125,8 +124,9 @@ export function mapPayloadPost(doc: PayloadPostDetail): PostDetail {
     id: doc.id,
     title: doc.title,
     slug: doc.slug,
-    category:
-      typeof doc.category === "object" && doc.category ? doc.category.name : "",
+    categories: (doc.categories ?? [])
+      .map((c) => (typeof c === "object" && c ? c.name : null))
+      .filter((name): name is string => Boolean(name)),
     readTime: doc.readTime,
     excerpt: doc.excerpt,
     thumbnail: