feat(issued-orders): make line items optional (issue by sections alone)

Items are no longer required on issued orders — an order can be issued
purely from its rich-text sections (Obsah).

- DocumentItemsEditor: opt-in allowEmpty prop lets the list go to zero
  rows (default false keeps the offers/invoices at-least-one-item rule).
- IssuedOrderDetail: allowEmpty on the editor; a saved order with no
  items reopens empty (not a blank starter row); submit guard now
  requires at least one item OR a non-empty section, not an item.
- PDF: with no items the billing heading, items table and total row are
  omitted entirely — a sections-only order shows only its Obsah.
- Service: finalize (draft->sent) and create-as-non-draft reject a
  completely blank order (no items AND no sections) with a Czech 400
  (empty_document); drafts may still be saved empty as WIP.
- Tests: sections-only finalize + blank-order rejection + sections-only
  PDF render; existing finalize fixtures given minimal content.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/routes/admin/issued-orders.ts

@@ -366,6 +366,8 @@ export default async function issuedOrdersRoutes(fastify: FastifyInstance) {
           return error(reply, "Dodavatel nenalezen", 400);
         if (order.error === "po_number_taken")
           return error(reply, "Číslo objednávky je již použito", 409);
+        if (order.error === "empty_document")
+          return error(reply, "Přidejte alespoň jednu položku nebo obsah", 400);
         return error(reply, "Neznámá chyba", 500);
       }
       await logAudit({
@@ -408,6 +410,8 @@ export default async function issuedOrdersRoutes(fastify: FastifyInstance) {
             `Neplatný přechod stavu z "${result.currentStatus}" na "${result.newStatus}"`,
             400,
           );
+        if (result.error === "empty_document")
+          return error(reply, "Přidejte alespoň jednu položku nebo obsah", 400);
         return error(reply, "Neznámá chyba", 500);
       }
       await logAudit({