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/blog/page.tsx

@@ -65,7 +65,7 @@ export default async function BlogOverviewPage() {
               </div>
               <div className="flex flex-col justify-center gap-3 p-8 sm:p-12 min-w-0">
                 <div className="flex items-center gap-2 font-semibold text-text-muted text-body-sm uppercase tracking-wide">
-                  <span>{featured.category}</span>
+                  <span>{featured.categories.join(", ")}</span>
                   <span>•</span>
                   <span>{featured.readTime} Min</span>
                   <span>•</span>
@@ -106,7 +106,7 @@ export default async function BlogOverviewPage() {
                   </div>
                   <div className="flex flex-col gap-2 min-w-0 flex-1">
                     <div className="flex items-center gap-2 font-semibold text-text-muted text-body-sm uppercase tracking-wide">
-                      <span>{post.category}</span>
+                      <span>{post.categories.join(", ")}</span>
                       <span>•</span>
                       <span>{post.readTime} Min</span>
                       <span>•</span>