feat(plot): printer driver properties and a roomier plot dialog

Properties… next to the printer used to open the CUPS web interface in a
browser. It now edits the driver options of the *selected* printer the
way LibreOffice's Properties… does, without leaving the plot dialog:

- CUPS platforms (Linux, any desktop; macOS): an in-line editor lists the
  printer's PPD options as `lpoptions -l` reports them — media type,
  colour mode, quality, duplex, trays, scaling — with readable choice
  labels, Reset / Cancel / Apply. Apply keeps only the choices that differ
  from the driver defaults, per printer, in the plot config, and every job
  for that printer passes them as `lp -o key=value`. Where CUPS cannot
  answer, the desktop's printer settings panel opens instead of a browser.
- Windows: the driver's document-properties sheet (`DocumentPropertiesW`
  with prompt, owned by the main window) and, on OK, the result stored as
  the user's printing preferences for that printer (`SetPrinter` level 9),
  which the job honours. `printui /e` (printing preferences) replaces the
  device-properties sheet as the fallback.

The desktop print portal was tried first and dropped: GTK 4's print
dialog loses the preselected printer when the default queue is listed
before it, so the dialog always opened on the wrong printer.

Dialog layout: the modal grows to 940×620; Printer / plotter and Paper
size share a line with their pickers (long queue and media names now fit);
Copies + Properties… and Custom… sit beneath; the custom-sheet editor
labels each margin (Left / Right, Top / Bottom) and shows the unit.

Also recognises `none_device`, AutoCAD's stored spelling for "no plotter",
and writes it back unchanged.

Tests: `lpoptions` parsing and keyword humanising, the editor flow (load,
stale answer ignored, set, apply, remember, reset, error, cancel), device
name round-trips including `none_device`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/io/plot_device.rs

@@ -22,8 +22,10 @@ use std::sync::{Arc, Mutex, OnceLock};
 
 /// AutoCAD's plotter configuration name for PDF output.
 pub const PDF_DEVICE_NAME: &str = "DWG To PDF.pc3";
-/// AutoCAD's spelling for "no plotter assigned".
+/// AutoCAD's spelling for "no plotter assigned" in the plot dialog…
 pub const NO_DEVICE_NAME: &str = "None";
+/// …and the name it actually stores in the drawing for that choice.
+const NO_DEVICE_STORED_NAME: &str = "none_device";
 /// The label older Open CAD Studio builds wrote into drawings for PDF output.
 /// Recognised on read so those drawings keep plotting to PDF, and always
 /// rewritten with [`PDF_DEVICE_NAME`] on save.
@@ -44,7 +46,7 @@ impl PlotDevice {
     /// The name AutoCAD would store for this device.
     pub fn canonical_name(&self) -> String {
         match self {
-            PlotDevice::None => NO_DEVICE_NAME.to_string(),
+            PlotDevice::None => NO_DEVICE_STORED_NAME.to_string(),
             PlotDevice::Pdf => PDF_DEVICE_NAME.to_string(),
             PlotDevice::Printer(name) => name.clone(),
         }
@@ -61,6 +63,10 @@ impl PlotDevice {
         if name.is_empty() || name.eq_ignore_ascii_case(NO_DEVICE_NAME) {
             return (PlotDevice::None, false);
         }
+        if name.eq_ignore_ascii_case(NO_DEVICE_STORED_NAME) {
+            // AutoCAD's own spelling: keep it rather than rewriting it.
+            return (PlotDevice::None, true);
+        }
         if name == LEGACY_PDF_LABEL {
             return (PlotDevice::Pdf, false);
         }
@@ -387,7 +393,8 @@ mod tests {
     #[test]
     fn canonical_names_follow_autocad() {
         assert_eq!(PlotDevice::Pdf.canonical_name(), "DWG To PDF.pc3");
-        assert_eq!(PlotDevice::None.canonical_name(), "None");
+        // What AutoCAD writes for "None" in its plot dialog.
+        assert_eq!(PlotDevice::None.canonical_name(), "none_device");
         assert_eq!(
             PlotDevice::Printer("L3270".into()).canonical_name(),
             "L3270"
@@ -401,6 +408,14 @@ mod tests {
             PlotDevice::from_stored_name("None"),
             (PlotDevice::None, false)
         );
+        assert_eq!(
+            PlotDevice::from_stored_name("none_device"),
+            (PlotDevice::None, true)
+        );
+        assert_eq!(
+            PlotDevice::from_stored_name("NONE_DEVICE"),
+            (PlotDevice::None, true)
+        );
         assert_eq!(
             PlotDevice::from_stored_name("Save to PDF file…"),
             (PlotDevice::Pdf, false)