refactor: fix all Low findings from FLAWS_REPORT audit

- Auth: TOTP params from config, JWT error logging, audit log failure
  logging, replaced_by_hash validation on token rotation
- Invoices: remove dead VAT code, consistent PDF permissions,
  WebP magic-byte detection, deduped exchange-rate fetches
- Orders/Offers: multipart limit from config, use paginated() helper,
  payment method from DB in PDF
- Projects: verify project exists before creating note
- Attendance: action_type enum validation, consistent local-time
  shift_date construction, holiday attendance in work fund,
  trips.view permission on last-km query
- Users: paginated() helper usage, remove duplicate dashboard keys,
  parallel currency conversion, single hashToken implementation
- Frontend: memoized customInput, reliable print onload, modal prop
  standardization (isOpen), ConfirmModal type icons, id===0 key
  fallback, Login useCallback, CompanySettings ConfirmModal,
  Attendance timeout cleanup, Dashboard memoization, beforeunload
  dirty-state warnings on Invoice/Offer/Order detail
- Schema: invoice_alert_log timestamp, config/env comment on
  Date.prototype.toJSON override
- Utils: exchange-rate inflight dedup

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

src/admin/pages/InvoiceDetail.tsx

@@ -383,6 +383,8 @@ export default function InvoiceDetail() {
   const [saving, setSaving] = useState(false);
   const [loading, setLoading] = useState(true);
   const [invoiceNumber, setInvoiceNumber] = useState("");
+  const initialSnapshotRef = useRef<string | null>(null);
+  const hasSetInitialSnapshot = useRef(false);
 
   const [customers, setCustomers] = useState<Customer[]>([]);
   const [customerSearch, setCustomerSearch] = useState("");
@@ -664,6 +666,32 @@ export default function InvoiceDetail() {
     if (isEdit) fetchDetail();
   }, [isEdit, fetchDetail]);
 
+  useEffect(() => {
+    if (loading) {
+      hasSetInitialSnapshot.current = false;
+      return;
+    }
+    if (!hasSetInitialSnapshot.current) {
+      initialSnapshotRef.current = JSON.stringify({ form, items });
+      hasSetInitialSnapshot.current = true;
+    }
+  }, [loading, form, items]);
+
+  const isDirty = useMemo(() => {
+    if (!initialSnapshotRef.current) return false;
+    return JSON.stringify({ form, items }) !== initialSnapshotRef.current;
+  }, [form, items]);
+
+  useEffect(() => {
+    if (!isDirty) return;
+    const handler = (e: BeforeUnloadEvent) => {
+      e.preventDefault();
+      e.returnValue = "";
+    };
+    window.addEventListener("beforeunload", handler);
+    return () => window.removeEventListener("beforeunload", handler);
+  }, [isDirty]);
+
   // ─── Due date calculation from issue date + days ───
   useEffect(() => {
     if (!form.issue_date) return;
@@ -826,6 +854,8 @@ export default function InvoiceDetail() {
           result.message ||
             (isEdit ? "Faktura byla uložena" : "Faktura byla vytvořena"),
         );
+        initialSnapshotRef.current = JSON.stringify({ form, items });
+        hasSetInitialSnapshot.current = true;
         if (isEdit) {
           fetchDetail();
         } else {