feat(documents)!: unify offers and issued orders end to end

One coherent pass over the two sibling document models, driven by a 3-lens
1:1 scan (~60 divergences inventoried) + user decisions. Migration adds
issued_orders.locked_by/locked_at and the issued_order_sections table
(mirror of scope_sections; applied to dev + test DBs).

Issued orders gained (offers as reference implementation):
- rich-text SECTIONS with CZ/EN titles, rendered on their own PDF page in
  the PO template's red style (shared DocumentSectionSchema, one-transaction
  create/update incl. items - fixes a torn-write bug)
- edit LOCKING (lock/heartbeat/unlock routes, 423 + holder name, 30s TTL =
  3 missed 10s heartbeats; locked_by enrichment on detail)
- archived-PDF serving: GET /:id/file reads the NAS copy (new
  readIssuedByNumber sweep) with live-render fallback + re-archive; offers'
  /file got the same fallback (kills the 'ulozte nabidku' dead end)
- NAS cleanup on delete, in-tx po_number uniqueness (409), collision-advancing
  number previews (also invoice previews), PUT returns assigned po_number

Offers hardened (issued as reference):
- VALID_TRANSITIONS enforced (no more numberless 'ordered' offers; invalidate
  follows the table; order creation only from active offers) +
  valid_transitions on detail
- explicit 400 instead of silent edits outside draft/active (mirrored on
  issued: explicit 400 replaced silent drops)
- customer existence check + Number(null)->0 clear bug fixed; delete of a
  linked offer -> 409 instead of P2003 500; error-token convention; Zod caps
  DB-aligned (desc 500/unit 20/number 50/project_code 100, isoDateString
  dates, ints for positions); audit old/new values + koncept fallbacks;
  list id tiebreaks; stats include trimmed; NAS delete dedupe

Frontend unification (6 new shared modules):
- components/document/{DocumentItemsEditor,SectionsEditor,LockBanner}
- hooks/{useDocumentLock,useUnsavedChangesGuard,useDocumentPdf(+list variant)}
- OfferDetail 1940->1100 lines, IssuedOrderDetail 1400->980: headline-only
  document number (form field removed), one form layout/readonly convention,
  view-permission opens read-only everywhere (issued's editable-for-viewers
  hole closed), server-driven transition buttons, dirty guard + Enter submit
  on both, useApiMutation everywhere (new opt-in envelope mode), fixed
  infinite spinner on failed detail fetch, draft PDFs hidden (no number yet)
- lists: supplier filter + count line on issued, Mena column dropped + mono
  numbers on offers, shared hardened per-row PDF flow (spinner, double-click
  guard, 401 close, blob cleanup), proper Czech quotes, real CTA empty
  states, query-lib cleanups (["offers","customers"] key, typed list rows,
  shared CurrencyAmount, retry:false on details)
- pdf-shared.ts: one escapeHtml/cleanQuillHtml(strict)/formatNum(NBSP)/
  formatCurrency/formatDate for all four PDF routes; offer PDF keeps its
  monochrome look (fractional qty fix: 1.5 no longer prints as 2); issued
  PDF language now comes from the document column

+49 tests (suite 364 -> 413). Each stage passed an independent review; final
cross-stage integration review verified the FE<->BE contracts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/hooks/useDocumentLock.ts

@@ -0,0 +1,81 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+import apiFetch from "../utils/api";
+
+/** Lock holder as enriched by the detail endpoints (offers, issued orders). */
+export interface DocumentLockInfo {
+  user_id: number;
+  username: string;
+  full_name: string;
+}
+
+/**
+ * Client side of the document edit lock shared by the offer / issued-order
+ * detail pages (extracted from OfferDetail). The server exposes the same
+ * trio under the entity base URL: POST `:base/lock`, `:base/heartbeat`,
+ * `:base/unlock` (lock expires after 30 s without a heartbeat — 3 missed
+ * 10 s beats, so background-tab interval throttling can't hand the lock away).
+ *
+ * Responsibilities:
+ * - `lockedBy` state — the page hydrates it from the detail response via
+ *   `setLockedBy(detail.locked_by ?? null)`.
+ * - `acquire()` — fire-and-forget lock acquisition; the page calls it from
+ *   its one-shot hydration effect when the doc is editable, the user holds
+ *   the edit permission and nobody else holds the lock.
+ * - 10 s heartbeat while `enabled` (and not locked by another user), with
+ *   hard-401 self-stop: a 401 here means the access token expired AND the
+ *   refresh failed (apiFetch retries recoverable 401s itself), so we stop
+ *   the interval instead of spamming a dead session every 10 s.
+ * - unlock on unmount/disable, with an AbortController so a stale in-flight
+ *   unlock can be cancelled by the next one.
+ */
+export function useDocumentLock({
+  baseUrl,
+  enabled,
+}: {
+  /** Entity endpoint base (e.g. `/api/admin/offers/42`); null in create mode. */
+  baseUrl: string | null;
+  /**
+   * External conditions for holding the lock: edit mode + editable status +
+   * edit permission. The hook adds `!isLockedByOther` itself.
+   */
+  enabled: boolean;
+}) {
+  const [lockedBy, setLockedBy] = useState<DocumentLockInfo | null>(null);
+  const heartbeatRef = useRef<ReturnType<typeof setInterval> | null>(null);
+  const unlockAbortRef = useRef<AbortController | null>(null);
+  const isLockedByOther = lockedBy !== null;
+
+  const acquire = useCallback(() => {
+    if (!baseUrl) return;
+    apiFetch(`${baseUrl}/lock`, { method: "POST" }).catch(() => {});
+  }, [baseUrl]);
+
+  // Heartbeat to keep the lock alive + release on unmount/disable.
+  useEffect(() => {
+    if (!baseUrl || !enabled || isLockedByOther) return;
+
+    heartbeatRef.current = setInterval(() => {
+      apiFetch(`${baseUrl}/heartbeat`, { method: "POST" })
+        .then((res) => {
+          if (res.status === 401 && heartbeatRef.current) {
+            clearInterval(heartbeatRef.current);
+            heartbeatRef.current = null;
+          }
+        })
+        .catch(() => {});
+    }, 10 * 1000); // every 10 seconds
+
+    return () => {
+      if (heartbeatRef.current) clearInterval(heartbeatRef.current);
+      if (unlockAbortRef.current) unlockAbortRef.current.abort();
+      const controller = new AbortController();
+      unlockAbortRef.current = controller;
+      apiFetch(`${baseUrl}/unlock`, {
+        method: "POST",
+        signal: controller.signal,
+      }).catch(() => {});
+    };
+  }, [baseUrl, enabled, isLockedByOther]);
+
+  return { lockedBy, isLockedByOther, setLockedBy, acquire };
+}