fix: Fix DatePicker initialization issue.

demo/Ursa.Demo/ViewModels/DatePickerDemoViewModel.cs

@@ -1,6 +1,28 @@
-namespace Ursa.Demo.ViewModels;
+using System;
+using System.ComponentModel;
+using CommunityToolkit.Mvvm.ComponentModel;
 
-public class DatePickerDemoViewModel
+namespace Ursa.Demo.ViewModels;
+
+public partial class DatePickerDemoViewModel: ObservableObject
 {
-    
+    [ObservableProperty] private DateTime? _selectedDate;
+    [ObservableProperty] private DateTime? _startDate;
+    [ObservableProperty] private DateTime? _endDate;
+
+    public DatePickerDemoViewModel()
+    {
+        SelectedDate = DateTime.Today;
+        StartDate = DateTime.Today;
+        EndDate = DateTime.Today.AddDays(7);
+    }
+
+    protected override void OnPropertyChanged(PropertyChangedEventArgs e)
+    {
+        base.OnPropertyChanged(e);
+        if (e.PropertyName == nameof(SelectedDate))
+        {
+            
+        }
+    }
 }