project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/ViewModels/Statistics.cs

@@ -0,0 +1,74 @@
+using System.Threading.Tasks;
+
+using Avalonia.Threading;
+
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace SourceGit.ViewModels
+{
+    public class Statistics : ObservableObject
+    {
+        public bool IsLoading
+        {
+            get => _isLoading;
+            private set => SetProperty(ref _isLoading, value);
+        }
+
+        public int SelectedIndex
+        {
+            get => _selectedIndex;
+            set
+            {
+                if (SetProperty(ref _selectedIndex, value))
+                    RefreshReport();
+            }
+        }
+
+        public Models.StatisticsReport SelectedReport
+        {
+            get => _selectedReport;
+            private set => SetProperty(ref _selectedReport, value);
+        }
+
+        public Statistics(string repo)
+        {
+            _repo = repo;
+
+            Task.Run(() =>
+            {
+                var result = new Commands.Statistics(_repo).Result();
+                Dispatcher.UIThread.Invoke(() =>
+                {
+                    _data = result;
+                    RefreshReport();
+                    IsLoading = false;
+                });
+            });
+        }
+
+        private void RefreshReport()
+        {
+            if (_data == null)
+                return;
+
+            switch (_selectedIndex)
+            {
+                case 0:
+                    SelectedReport = _data.Year;
+                    break;
+                case 1:
+                    SelectedReport = _data.Month;
+                    break;
+                default:
+                    SelectedReport = _data.Week;
+                    break;
+            }
+        }
+
+        private readonly string _repo = string.Empty;
+        private bool _isLoading = true;
+        private Models.Statistics _data = null;
+        private Models.StatisticsReport _selectedReport = null;
+        private int _selectedIndex = 0;
+    }
+}