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/commands/display.rs

@@ -358,12 +358,13 @@ impl OpenCADStudio {
             }
 
             // ── Plot / Page Setup ──────────────────────────────────────────
-            "PLOT" | "EXPORT" | "EXPORTPDF" => {
-                return Some(Task::done(Message::PlotExport));
+            // PLOT / PRINT open the full plot dialog (printer, paper, scale,
+            // options); EXPORT / EXPORTPDF stay a direct PDF export.
+            "PLOT" | "PRINT" => {
+                return Some(Task::done(Message::PlotDialogOpen));
             }
-            // PRINT — send current layout to the system default printer.
-            "PRINT" => {
-                return Some(Task::done(Message::PrintToPrinter));
+            "EXPORT" | "EXPORTPDF" => {
+                return Some(Task::done(Message::PlotExport));
             }
             // PLOTSTYLE — load or clear CTB/STB plot style table
             cmd if cmd == "PLOTSTYLE" || cmd.starts_with("PLOTSTYLE ") => {
@@ -509,11 +510,9 @@ impl OpenCADStudio {
                 }
             }
 
-            // Model space now has its own dialog section (window-plot format /
-            // orientation / pick), so PAGESETUP opens there too, not just on
-            // paper-space layouts.
+            // PAGESETUP is folded into the unified plot dialog.
             "PAGESETUP" => {
-                return Some(Task::done(Message::PageSetupOpen));
+                return Some(Task::done(Message::PlotDialogOpen));
             }
 
             // ── Recognized commands whose full implementation is pending ─────────