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/components/Blog.tsx

@@ -47,7 +47,7 @@ export async function Blog() {
           <div className="flex flex-col justify-between gap-4 py-4 px-4 sm:pr-4 sm:pl-4 sm:col-span-5 min-w-0">
             <div className="flex flex-col gap-3">
               <div className="flex gap-2 items-center font-semibold text-text-muted text-body uppercase whitespace-nowrap">
-                <span>{featured.category}</span>
+                <span>{featured.categories.join(", ")}</span>
                 <span>•</span>
                 <span>{featured.readTime} Min</span>
               </div>
@@ -96,7 +96,7 @@ export async function Blog() {
               <div className="flex flex-col justify-between gap-4 py-3 px-4 sm:col-span-3 min-w-0 text-text-primary">
                 <div className="flex flex-col gap-3">
                   <div className="flex gap-2 items-center font-semibold text-text-muted text-body uppercase whitespace-nowrap">
-                    <span>{post.category}</span>
+                    <span>{post.categories.join(", ")}</span>
                     <span>•</span>
                     <span>{post.readTime} Min</span>
                   </div>