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

@@ -82,7 +82,7 @@
             <PathIcon Data="{StaticResource Icon.WinMin}" Width="10" Height="10"/>
           </Button>
           <Button Classes="title-ctrl" Click="OnToggleMax">
-            <PathIcon Data="{StaticResource Icon.WinMax}" Width="10" Height="10"/>
+            <PathIcon x:Name="MaxIcon" Data="{StaticResource Icon.WinMax}" Width="10" Height="10"/>
           </Button>
           <Button Classes="title-ctrl close" Click="OnClose">
             <PathIcon Data="{StaticResource Icon.WinClose}" Width="10" Height="10"/>