fix(ui): manual modal dragging, maximize/restore icon, day-toggle style

- Drive modal title-bar dragging manually via pointer capture + Window.Position;
  Avalonia 12's BeginMoveDrag and VisualRoot-as-Window cast no longer work
  (VisualRoot is a TopLevelHost). Applies to ModalShell and WorktreeModalView.
- Toggle the MainWindow maximize button between maximize/restore glyphs on
  WindowState changes (adds Icon.WinRestore geometry).
- Add the ToggleButton.day-toggle style used by the Prime weekday picker row.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml.cs

@@ -19,6 +19,21 @@ public partial class MainWindow : Window
         InitializeComponent();
         KeyDown += OnWindowKeyDown;
         DataContextChanged += OnDataContextChanged;
+        UpdateMaxIcon();
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+        if (change.Property == WindowStateProperty)
+            UpdateMaxIcon();
+    }
+
+    private void UpdateMaxIcon()
+    {
+        var key = WindowState == WindowState.Maximized ? "Icon.WinRestore" : "Icon.WinMax";
+        if (this.TryFindResource(key, out var geometry) && geometry is Geometry g)
+            MaxIcon.Data = g;
     }
 
     private void OnDataContextChanged(object? sender, EventArgs e)