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/TaskCreateModal.jsx

@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from 'preact/hooks';
 import { createTask, fetchTaskTemplates, getStatusLabel } from '../../stores/taskStore';
-import { formatEntityCode, formatPurposeLabel } from '../../lib/displayIds';
+import { formatPurposeLabel, formatWorkOrderDisplayCode } from '../../lib/displayIds';
 import ModalShell from '../ui/ModalShell';
 
 const STATUS_OPTIONS = ['aktivan', 'servis', 'zavrsen', 'neaktivan'];
@@ -25,6 +25,10 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
         () => templates.find((template) => String(template.id) === String(templateId)) || null,
         [templates, templateId]
     );
+    const availableWorkOrders = useMemo(
+        () => workOrders.filter((workOrder) => String(workOrder?.status || '').toLowerCase() !== 'closed'),
+        [workOrders]
+    );
 
     useEffect(() => {
         if (!open) {
@@ -235,9 +239,9 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
                                 className="rounded-lg border border-border-hairline bg-canvas-base px-3 py-2 text-text-main disabled:opacity-50"
                             >
                                 <option value="">— Bez povezanog naloga —</option>
-                                {workOrders.map((workOrder) => (
+                                {availableWorkOrders.map((workOrder) => (
                                     <option key={workOrder.id} value={String(workOrder.id)}>
-                                        {formatEntityCode('PN', workOrder.id)} • {workOrder.craneLabel || workOrder.craneInfo?.registration_number || '-'} • {formatPurposeLabel(workOrder.purpose) || '-'}
+                                        {formatWorkOrderDisplayCode(workOrder)} • {workOrder.craneLabel || workOrder.craneInfo?.registration_number || '-'} • {formatPurposeLabel(workOrder.purpose) || '-'}
                                     </option>
                                 ))}
                             </select>
@@ -246,7 +250,7 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
                                     Kod template unosa nalog nije obavezan i backend ga neće automatski povezati.
                                 </span>
                             )}
-                            {workOrders.length === 0 && (
+                            {availableWorkOrders.length === 0 && (
                                 <span className="text-[11px] text-amber-600">
                                     Nema dostupnih naloga za odabranu dizalicu.
                                 </span>