feat(rechnung): branded invoice with VAT ID and per-rate grouping

The invoice PDF and the invoice page now share one branded layout:
logo (uploaded brand logo, otherwise the drawn honeycomb mark) and
wordmark in the header, honey accents, a forest-green table header,
a meta box with invoice number, date, order number and VAT ID, and a
footer with address, contact, VAT ID and page numbers on every page.

A mixed cart (e.g. honey at 7 % and a beeswax candle at 19 %) lists
its items in one block per VAT rate with a subtotal each. Next to the
totals, a VAT table shows net, VAT and gross per rate plus a sum row.
Long invoices continue on further pages with the table header repeated.

The VAT identification number is a new company field
(company.vatId), editable in Admin > Inhalte, defaulting to the
number already stated in the imprint.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

src/lib/site-config.ts

@@ -31,6 +31,12 @@ export const siteConfig = {
    * Zeile jetzt ueberall weggelassen (kontakt/page.tsx, impressum/page.tsx).
    */
   phone: "",
+  /**
+   * Umsatzsteuer-Identifikationsnummer (§ 27a UStG). Steht auf jeder
+   * Rechnung (§ 14 Abs. 4 Nr. 2 UStG) und ist im Admin unter Inhalte >
+   * Firmendaten pflegbar; dieser Wert ist nur die Vorgabe.
+   */
+  vatId: "DE366479158",
   address: {
     street: "Buhlweg 10",
     zip: "86450",