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/TaskServiceRecordsModal.jsx

@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from 'preact/hooks';
 import ModalShell from '../ui/ModalShell';
-import { formatEntityCode } from '../../lib/displayIds';
+import { formatWorkOrderDisplayCode } from '../../lib/displayIds';
 import { getStatusLabel } from '../../stores/taskStore';
 
 const STATUS_OPTIONS = ['aktivan', 'servis', 'spreman_za_zavrsetak', 'zavrsen', 'neaktivan'];
@@ -301,7 +301,7 @@ export default function TaskServiceRecordsModal({
                                 {currentWorkOrder && !showWorkOrderEditor ? (
                                     <div className="flex flex-wrap items-center justify-between gap-2 rounded border border-border-hairline bg-canvas-elevated px-3 py-2">
                                         <p className="text-sm text-text-main">
-                                            {formatEntityCode('PN', currentWorkOrder.id)} • {currentWorkOrder.craneLabel || currentWorkOrder.craneInfo?.registration_number || '-'} • {currentWorkOrder.status || '-'}
+                                            {formatWorkOrderDisplayCode(currentWorkOrder)} • {currentWorkOrder.craneLabel || currentWorkOrder.craneInfo?.registration_number || '-'} • {currentWorkOrder.status || '-'}
                                         </p>
                                         <button
                                             type="button"
@@ -322,7 +322,7 @@ export default function TaskServiceRecordsModal({
                                             <option value="">— Odaberi putni nalog —</option>
                                             {sortedWorkOrders.map((workOrder) => (
                                                 <option key={workOrder.id} value={String(workOrder.id)}>
-                                                    {formatEntityCode('PN', workOrder.id)} • {workOrder.craneLabel || workOrder.craneInfo?.registration_number || '-'} • {workOrder.status || '-'}
+                                                    {formatWorkOrderDisplayCode(workOrder)} • {workOrder.craneLabel || workOrder.craneInfo?.registration_number || '-'} • {workOrder.status || '-'}
                                                 </option>
                                             ))}
                                         </select>