feat: dodaj display_code polje za putne naloge (MT070626 format)

- Dodano polje 'display_code' na WorkOrder model (format: 2 inicijala + ddmmyy)
- Migracije za kreiranje polja i backfill postojećih naloga
- Ažuriran WorkOrderSerializer za validaciju i serijalizaciju display_code
- Backend tasks za auto-generiranje display_code pri kreiranju
- Frontend komponente ažurirane za prikaz display_code umjesto UUID:
  * FleetDashboardShell tablica putnih naloga
  * WorkOrderDetailModal
  * TaskCreateModal
  * Sve PDF stranice
- displayIds.js utility za formatiranjeDisplay ID-eva
- Admin interface prikazuje display_code

frontend/src/components/dashboard/WorkOrderInvoicesPdfPage.jsx

@@ -11,7 +11,7 @@ import {
 } from '../../stores/fleetDashboardStore';
 import { $accessToken, $authReady, hydrateAuthFromStorage } from '../../stores/authStore';
 import { fetchNotifications, connectNotifications } from '../../stores/notificationStore';
-import { formatEntityCode } from '../../lib/displayIds';
+import { formatWorkOrderDisplayCode } from '../../lib/displayIds';
 import { resolveMediaUrl, useAuthenticatedMediaSources } from './WorkOrderImageCarousel';
 import TaskWorkHoursTableModal from './TaskWorkHoursTableModal';
 
@@ -122,7 +122,7 @@ export default function WorkOrderInvoicesPdfPage() {
         if (!workOrder) {
             return 'Računi putnog naloga';
         }
-        return `${formatEntityCode('PN', workOrder.id)}`;
+        return formatWorkOrderDisplayCode(workOrder);
     }, [workOrder]);
 
     const mediaPaths = useMemo(() => {