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/models.py

@@ -287,6 +287,13 @@ class WorkOrder(BaseModel):
     location = models.CharField(max_length=255, blank=True, verbose_name=_("Lokacija odredišta"))
     travel_start_at = models.DateTimeField(null=True, blank=True, verbose_name=_("Početak puta"))
     travel_end_at = models.DateTimeField(null=True, blank=True, verbose_name=_("Kraj puta"))
+    display_code = models.CharField(
+        max_length=8,
+        blank=True,
+        db_index=True,
+        verbose_name=_("Prikaz putnog naloga"),
+        help_text=_("Format: MT070626 (2 slova + 6 znamenki)."),
+    )
     purpose = models.CharField(max_length=50, choices=PURPOSE_CHOICES, blank=True, verbose_name=_("Svrha"))
     notes = models.TextField(blank=True, verbose_name=_("Bilješke"))
     status = models.CharField(max_length=20, choices=STATUS_CHOICES, default='open', verbose_name=_("Status"))
@@ -322,6 +329,12 @@ class WorkOrder(BaseModel):
 
     def clean(self):
         super().clean()
+        if self.display_code:
+            self.display_code = str(self.display_code).strip().upper()
+            if not re.fullmatch(r"[A-Z]{2}[0-9]{6}", self.display_code):
+                raise ValidationError({
+                    "display_code": _("Prikaz putnog naloga mora biti u formatu MT070626 (2 slova + 6 znamenki).")
+                })
         if self.travel_start_at and self.travel_end_at and self.travel_end_at < self.travel_start_at:
             raise ValidationError({"travel_end_at": _("Kraj puta ne može biti prije početka puta.")})