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>

frontend/src/components/dashboard/TaskCreateModal.jsx

@@ -10,6 +10,7 @@ const INITIAL_FORM = {
     description: '',
     status: 'aktivan',
     work_order: '',
+    scheduled_date: '',
 };
 
 export default function TaskCreateModal({ open, workOrders = [], contextCrane = null, onClose, onSuccess }) {
@@ -50,6 +51,7 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
             description: selectedTemplate.description || '',
             status: 'aktivan',
             work_order: '',
+            // scheduled_date is kept so user can set it in template mode too
         }));
     }, [selectedTemplate]);
 
@@ -98,6 +100,7 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
                 template_id: templateId,
                 vehicle: contextCrane.id,
                 work_order: null,
+                scheduled_date: form.scheduled_date || null,
             };
         } else {
             const title = String(form.title || '').trim();
@@ -115,6 +118,7 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
                 status: form.status || 'aktivan',
                 vehicle: contextCrane.id,
                 work_order: form.work_order || null,
+                scheduled_date: form.scheduled_date || null,
             };
         }
 
@@ -231,6 +235,17 @@ export default function TaskCreateModal({ open, workOrders = [], contextCrane =
                         </label>
 
                         <label className="flex flex-col gap-1 text-sm">
+                            <span className="font-medium text-text-main">Planirani datum izvršenja</span>
+                            <input
+                                type="date"
+                                value={form.scheduled_date}
+                                onInput={(event) => setField('scheduled_date', event.currentTarget.value)}
+                                disabled={submitting}
+                                className="rounded-lg border border-border-hairline bg-canvas-base px-3 py-2 text-text-main disabled:opacity-50"
+                            />
+                        </label>
+
+                        <label className="flex flex-col gap-1 text-sm sm:col-span-2">
                             <span className="font-medium text-text-main">Povezani putni nalog</span>
                             <select
                                 value={form.work_order}