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/view/mod.rs

@@ -1466,7 +1466,7 @@ impl OpenCADStudio {
             PluginManager => (520, 460),
             UpdateNotice => (560, 460),
             Layers => (900, 360),
-            PageSetup => (520, 460),
+            Plot => (660, 540),
             LayoutManager => (640, 320),
             Plotstyle => (780, 540),
             TextStyle => (860, 480),