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/Modals/WorktreeModalView.axaml

@@ -28,7 +28,10 @@
 
       <!-- Title strip -->
       <Border DockPanel.Dock="Top" Height="36"
-              PointerPressed="OnTitleBarPressed">
+              Background="Transparent"
+              PointerPressed="OnTitleBarPressed"
+              PointerMoved="OnTitleBarMoved"
+              PointerReleased="OnTitleBarReleased">
         <Grid ColumnDefinitions="*,Auto" Margin="14,0">
           <TextBlock Grid.Column="0" Text="Worktree" VerticalAlignment="Center"
                      FontFamily="{DynamicResource MonoFont}" FontSize="{StaticResource FontSizeBody}"

src/ClaudeDo.Ui/Views/Modals/WorktreeModalView.axaml.cs

@@ -66,9 +66,31 @@ public partial class WorktreeModalView : Window
         e.Handled = true;
     }
 
+    private PixelPoint _dragStartScreen;
+    private PixelPoint _dragStartPos;
+    private bool _dragging;
+
     private void OnTitleBarPressed(object? sender, PointerPressedEventArgs e)
     {
-        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
-            BeginMoveDrag(e);
+        if (!e.GetCurrentPoint(this).Properties.IsLeftButtonPressed) return;
+        _dragStartScreen = this.PointToScreen(e.GetPosition(this));
+        _dragStartPos = Position;
+        _dragging = true;
+        e.Pointer.Capture(sender as IInputElement);
+    }
+
+    private void OnTitleBarMoved(object? sender, PointerEventArgs e)
+    {
+        if (!_dragging || !e.GetCurrentPoint(this).Properties.IsLeftButtonPressed) return;
+        var cur = this.PointToScreen(e.GetPosition(this));
+        Position = new PixelPoint(
+            _dragStartPos.X + (cur.X - _dragStartScreen.X),
+            _dragStartPos.Y + (cur.Y - _dragStartScreen.Y));
+    }
+
+    private void OnTitleBarReleased(object? sender, PointerReleasedEventArgs e)
+    {
+        _dragging = false;
+        e.Pointer.Capture(null);
     }
 }