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-pdf.ts

@@ -752,8 +752,11 @@ ${indentCSS}
     </tr>
   </table>
 
-  <!-- Polozky -->
-  <div class="billing-label">${escapeHtml(order.order_text || t.billing)}</div>
+  <!-- Polozky — an order may be issued by sections alone; with no items the
+       heading, items table and total row are omitted entirely. -->
+  ${
+    items.length > 0
+      ? `<div class="billing-label">${escapeHtml(order.order_text || t.billing)}</div>
   <table class="items">
     <thead>
       <tr>
@@ -779,7 +782,9 @@ ${indentCSS}
         <span class="value">${formatCurrency(total, currency)}</span>
       </div>
     </div>
-  </div>
+  </div>`
+      : ""
+  }
 
   ${scopeHtml}