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/lib/displayIds.js

@@ -12,6 +12,24 @@ export function formatEntityCode(prefix, id, size = 8) {
     return `${prefix}-${short}`;
 }
 
+export function generateWorkOrderDisplayCode(firstName, lastName, dateValue = new Date()) {
+    const firstInitial = String(firstName || '').trim().charAt(0).toUpperCase();
+    const lastInitial = String(lastName || '').trim().charAt(0).toUpperCase();
+    if (!firstInitial || !lastInitial) return '';
+    const parsedDate = dateValue instanceof Date ? dateValue : new Date(dateValue);
+    if (Number.isNaN(parsedDate.getTime())) return '';
+    const day = String(parsedDate.getDate()).padStart(2, '0');
+    const month = String(parsedDate.getMonth() + 1).padStart(2, '0');
+    const year = String(parsedDate.getFullYear()).slice(-2);
+    return `${firstInitial}${lastInitial}${day}${month}${year}`;
+}
+
+export function formatWorkOrderDisplayCode(workOrder) {
+    const displayCode = String(workOrder?.display_code || '').trim().toUpperCase();
+    if (displayCode) return displayCode;
+    return '-';
+}
+
 export function formatPurposeLabel(value) {
     const normalized = String(value || '').trim().toLowerCase();
     if (!normalized) return '';