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/admin/pages/IssuedOrderDetail.tsx

@@ -301,7 +301,9 @@ export default function IssuedOrderDetail() {
             // Issued orders have no Sleva column; keep the shared item shape happy.
             discount: 0,
           }))
-        : [emptyDocumentItem()];
+        : // Issued orders may be issued by sections alone — a saved order with
+          // no items reopens with an empty list (not a blank starter row).
+          [];
     setItems(mappedItems);
 
     const mappedSections =
@@ -401,8 +403,18 @@ export default function IssuedOrderDetail() {
     const newErrors: Record<string, string> = {};
     if (!form.supplier_id) newErrors.supplier_id = "Vyberte dodavatele";
     if (!form.order_date) newErrors.order_date = "Zadejte datum";
-    if (items.length === 0 || items.every((i) => !i.description.trim())) {
-      newErrors.items = "Přidejte alespoň jednu položku";
+    // Items are optional (an order may be issued by sections alone), but a
+    // completely blank order must not be finalizable: require at least one
+    // item with a description OR one non-empty section (title or content).
+    const hasItem = items.some((i) => i.description.trim());
+    const hasSection = sections.some(
+      (s) =>
+        (s.title_cz || "").trim() ||
+        (s.title || "").trim() ||
+        (s.content || "").replace(/<[^>]*>/g, "").trim(),
+    );
+    if (!hasItem && !hasSection) {
+      newErrors.items = "Přidejte alespoň jednu položku nebo obsah";
     }
     setErrors(newErrors);
     if (Object.keys(newErrors).length > 0) return;
@@ -899,6 +911,7 @@ export default function IssuedOrderDetail() {
           currency={form.currency}
           readOnly={!editable}
           error={errors.items}
+          allowEmpty
         />
 
         {/* Rich-text PDF sections — the free-form PDF content lives here */}