feat: dodaj scheduled_date na Task i kalendar widget

- Novo polje Task.scheduled_date (DateField, nullable, db_index) za planiranje
- Migracija 0009_task_scheduled_date
- Celery task notify_upcoming_tasks: dnevno u 08:00 šalje Pusher + email za sutrašnje zadatke
- TaskCalendarWidget: react-spring slide panel s lijeve strane, grid prikaz mjeseca, dot indikatori, klik otvara TaskServiceRecordsModal
- FleetDashboardShell: import + render kalendara, Planirano stupac u tablici zadataka
- TaskCreateModal + TaskServiceRecordsModal: scheduled_date input i prikaz
- Admin + serializer + views: scheduled_date filter i prikaz
- 47 backend testova prolaze (uključuje 8 novih za notify_upcoming_tasks)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/task_management/views.py

@@ -27,6 +27,9 @@ class TaskViewSet(viewsets.ModelViewSet):
         vehicle_id = self.request.query_params.get('vehicle')
         if vehicle_id:
             qs = qs.filter(vehicle_id=vehicle_id)
+        scheduled_date = self.request.query_params.get('scheduled_date')
+        if scheduled_date:
+            qs = qs.filter(scheduled_date=scheduled_date)
         return qs
 
     def perform_create(self, serializer):