feat(plot): unified plot/print dialog, retire Page Setup

Replace the old Page Setup panel with a single OCS-styled plot/print
dialog (dropdowns for printer, paper, orientation, rotation, plot area,
scale, quality, shaded plot; checkboxes for plot options). PLOT / PRINT
/ PAGESETUP all open it.

- Printer output via lp: -d printer, -n copies, -o ColorModel=Gray,
  print-quality, Resolution; Windows printto verb; list_printers via
  lpstat. Preview opens a temp PDF in the OS viewer (never a save
  dialog). PDF export reuses the tested pipeline.
- Window pick returns to the new dialog (was the old Page Setup) and the
  clipped window plot drives PDF / printer / preview.
- Remember the user's print preferences across sessions in
  config/plot.txt (printer, copies, quality, output options, area,
  scale); drawing-specific fields (offset/center/rotation) seed from the
  layout. Default plot area Window, scale Fit.
- Delete src/ui/window/page_setup.rs and its ModalKind / Message /
  buffer state; fold on_page_setup_commit into apply_plot_page_settings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/command_driver.rs

@@ -1187,10 +1187,11 @@ impl OpenCADStudio {
                     self.command_line.push_output(&format!(
                         "Plot window: {x0:.2},{y0:.2} to {x1:.2},{y1:.2}"
                     ));
-                    // Pick window closed Page Setup so the viewport could
-                    // receive the two clicks — bring it back for scale
-                    // review and the "Plot window → PDF" button.
-                    self.active_modal = Some(super::ModalKind::PageSetup);
+                    // Pick window closed the plot dialog so the viewport could
+                    // receive the two clicks — bring the dialog back with the
+                    // window now active.
+                    self.plot_dialog.area = "Window".to_string();
+                    self.active_modal = Some(super::ModalKind::Plot);
                 } else {
                     let block_handle = self.tabs[i].scene.current_layout_block_handle_pub();
                     let doc = &mut self.tabs[i].scene.document;