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/checkoutDraft.ts

@@ -36,6 +36,9 @@ export type CheckoutDraft = {
   shippingZip: string;
   shippingCity: string;
   shippingCountry: string;
+  shippingCompanyName: string;
+  shippingContactEmail: string;
+  shippingContactPhone: string;
   newsletterOptIn: boolean;
   shippingMethodId: number | null;
   paymentMethodId: number | null;