security: fix all Medium findings from FLAWS_REPORT audit

- Auth: TOTP replay protection with counter tracking, constant-time
  backup code comparison, atomic lockout increment, per-token logout
- Invoices/PDFs: net-based VAT calculation, dangerous URL scheme
  stripping in cleanQuillHtml, orders-pdf error handling
- Orders: reject item changes on status transition, cascading
  delete cleanup, take:1 with orderBy
- Projects: atomic rename collision handling, MIME/extension
  validation, empty customer name rejection
- Attendance: Czech public holiday awareness in frontend fund
  calculation, leave_hours 0 handling, invalid date NaN guard,
  bounded per-month queries in workfund
- Users/Admin: profile audit logging + password validation, session
  revocation guard, session ID validation, dashboard DB aggregation,
  soft-deleted record protection in scope templates
- Frontend: FormField label linkage, Pagination ARIA, error
  handling in OrderConfirmationModal, 401 propagation, GPS emoji
  hidden from screen readers, table sort state fix, geolocation
  race/abort cleanup, Leaflet popup DOM safety, Vehicles toggleActive
  minimal body, CompanySettings ref mutation fix, OfferDetail unlock
  abort, AttendanceBalances combined fetches
- Utils: env validation, Puppeteer concurrency mutex, invoice alert
  cron cleanup on shutdown, body limit alignment, TOTP error logging,
  trustProxy from env, symlink rejection, rate cache Map usage

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/admin/pages/OfferDetail.tsx

@@ -334,6 +334,7 @@ export default function OfferDetail() {
     full_name: string;
   } | null>(null);
   const heartbeatRef = useRef<ReturnType<typeof setInterval> | null>(null);
+  const unlockAbortRef = useRef<AbortController | null>(null);
 
   useModalLock(showOrderModal);
 
@@ -451,10 +452,14 @@ export default function OfferDetail() {
 
     return () => {
       if (heartbeatRef.current) clearInterval(heartbeatRef.current);
+      if (unlockAbortRef.current) unlockAbortRef.current.abort();
       // Release lock on unmount
-      apiFetch(`${API_BASE}/offers/${id}/unlock`, { method: "POST" }).catch(
-        () => {},
-      );
+      const controller = new AbortController();
+      unlockAbortRef.current = controller;
+      apiFetch(`${API_BASE}/offers/${id}/unlock`, {
+        method: "POST",
+        signal: controller.signal,
+      }).catch(() => {});
     };
   }, [isEdit, id, isLockedByOther, isInvalidated]);