feat: plot a model-space window to PDF

Pick a rectangular window in model space with the existing PLOTWINDOW command (now enabled in model space, previously paper-space only) and export it to a PDF at a chosen ISO paper size (A4-A0), orientation and scale (fit or ratio). The window is a hard clip boundary. Adds paper-size presets, a window-to-sheet transform, scale + clip support in the PDF exporter, and Page Setup controls (format/orientation/pick window).

src/modules/view/plot_window.rs

@@ -1,7 +1,7 @@
 // PLOTWINDOW command — pick two corners to define the plot window area.
 //
-// Works only in paper space layouts. After picking P1 and P2, writes the
-// window to the layout's PlotSettings (PlotType::Window).
+// In paper space it writes the layout's PlotSettings (PlotType::Window); in
+// model space the host stores the window for the plot dialog.
 
 use crate::command::{CadCommand, CmdResult};
 use crate::scene::model::wire_model::WireModel;