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

backend/modules/fleet/tasks.py

@@ -18,6 +18,13 @@ logger = logging.getLogger(__name__)
 register_unicode_fonts()
 
 
+def _work_order_display_code(work_order):
+    normalized = str(getattr(work_order, 'display_code', '') or '').strip().upper()
+    if normalized:
+        return normalized
+    return 'NALOG'
+
+
 @shared_task(bind=True, max_retries=3, default_retry_delay=60)
 def send_notification_email(self, notification_id):
     """
@@ -95,6 +102,7 @@ def _build_work_order_invoices_pdf(work_order):
     chassis_hours = str(getattr(work_order.vehicle, "chassis_working_hours", "-") or "-")
     mileage = str(getattr(work_order.vehicle, "current_mileage", "-") or "-")
     generated_date = timezone.localtime(timezone.now()).strftime("%d.%m.%Y")
+    display_code = _work_order_display_code(work_order)
 
     def draw_header_footer(page_num):
         draw_standard_header_footer(
@@ -107,7 +115,7 @@ def _build_work_order_invoices_pdf(work_order):
             upgrade_hours=upgrade_hours,
             chassis_hours=chassis_hours,
             mileage=mileage,
-            work_order_number=str(work_order.pk),
+            work_order_number=display_code,
             generated_date=generated_date,
             report_title="Računi putnog naloga",
             page_size=A4,
@@ -122,7 +130,7 @@ def _build_work_order_invoices_pdf(work_order):
     y -= 8
 
     pdf.setFont("Vera-Bold", 13)
-    pdf.drawString(margin, y, f"Računi putnog naloga WO-{work_order.pk}")
+    pdf.drawString(margin, y, f"Računi putnog naloga {display_code}")
     y -= 20
     pdf.setFont("Vera", 10)
     pdf.drawString(margin, y, f"Datum naloga: {work_order.date}")
@@ -202,7 +210,7 @@ def build_work_order_invoices_pdf_task(work_order_id):
 
     pdf_bytes = _build_work_order_invoices_pdf(work_order)
     pdf_b64 = base64.b64encode(pdf_bytes).decode('ascii')
-    filename = f"{work_order.pk}.work-order-invoices.pdf"
+    filename = f"{_work_order_display_code(work_order)}.work-order-invoices.pdf"
     return {
         "filename": filename,
         "pdf_base64": pdf_b64,
@@ -259,7 +267,7 @@ def build_work_order_pdf_cached_task(generated_pdf_id):
         else:
             pdf_bytes = _build_work_order_pdf(work_order)
 
-        filename = generated.filename or f"{work_order.pk}.{generated.pdf_type}.pdf"
+        filename = generated.filename or f"{_work_order_display_code(work_order)}.{generated.pdf_type}.pdf"
         generated.file.save(filename, ContentFile(pdf_bytes), save=False)
         generated.status = 'ready'
         generated.error_message = ''
@@ -269,7 +277,7 @@ def build_work_order_pdf_cached_task(generated_pdf_id):
             NotificationService.create_notification(
                 recipient=generated.requested_by,
                 title="PDF spreman",
-                message=f"PDF dokument je uspješno generiran za WO-{work_order.pk}.",
+                message=f"PDF dokument je uspješno generiran za putni nalog {_work_order_display_code(work_order)}.",
                 level="success",
                 send_email=False,
                 metadata={
@@ -294,7 +302,7 @@ def build_work_order_pdf_cached_task(generated_pdf_id):
             NotificationService.create_notification(
                 recipient=generated.requested_by,
                 title="Greška kod PDF-a",
-                message=f"Generiranje PDF dokumenta nije uspjelo za WO-{work_order.pk}.",
+                message=f"Generiranje PDF dokumenta nije uspjelo za putni nalog {_work_order_display_code(work_order)}.",
                 level="warning",
                 send_email=False,
                 metadata={