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/CompanySettings.tsx

@@ -85,7 +85,6 @@ export default function CompanySettings({
     vat_id: "",
   });
   const [customFields, setCustomFields] = useState<CustomField[]>([]);
-  const customFieldKeyCounter = useRef(0);
   const [fieldOrder, setFieldOrder] = useState<string[]>([
     ...DEFAULT_FIELD_ORDER,
   ]);
@@ -197,9 +196,17 @@ export default function CompanySettings({
         const cf =
           Array.isArray(d.custom_fields) && d.custom_fields.length > 0
             ? d.custom_fields.map(
-                (f: { name: string; value: string; showLabel?: boolean }) => ({
+                (
+                  f: {
+                    name: string;
+                    value: string;
+                    showLabel?: boolean;
+                    _key?: string;
+                  },
+                  i: number,
+                ) => ({
                   ...f,
-                  _key: `cf-${++customFieldKeyCounter.current}`,
+                  _key: f._key || `cf-${Date.now()}-${i}`,
                 }),
               )
             : [];
@@ -716,7 +723,7 @@ export default function CompanySettings({
                         name: "",
                         value: "",
                         showLabel: true,
-                        _key: `cf-${++customFieldKeyCounter.current}`,
+                        _key: `cf-${Date.now()}`,
                       },
                     ])
                   }