fix(print): send Windows plots to the chosen printer, name the default, report failures

On Windows a plot with no printer chosen went to the PDF application's
bare "print" verb — whatever that application considered its default —
and a viewer without a "printto" verb (Edge, store readers) silently
dropped the printer the dialog had chosen. The dialog also never named the
system default, and a failed printer enumeration left only "Default" and
"PDF" with the reason written to a console nobody sees.

- The default printer is looked up by name (GetDefaultPrinterW) both for
  the dialog's "System default printer (…)" entry and before every job.
- A pure route decision (plan_print_route) resolves "Default" to that
  name and uses the PDF application only when it registered a "printto"
  verb for .pdf (AssocQueryStringW); otherwise the job goes straight to
  the spooler through the existing GDI path. The report says which:
  "Sent to printer: NAME (direct print | via the PDF application)".
- list_printers reports the system's own error; the plot dialog shows it
  under the printer row and the command line warns once. A desktop without
  CUPS is an empty list, not an error.
- PRINTERS command: default printer, listed printers (or the error) and,
  on Windows, the print route — a report to paste into an issue.
- Properties… on the Default entry opens that printer's preferences sheet.
- GDI path: the driver's dmSize is kept (its private data lives behind
  it), the print raster is no longer memoised for the session, and a
  spooled temporary PDF is removed.

Tests: route planning (named / blank / no default / printto or not), the
PRINTERS command; the two Windows-only GDI tests pass the resolved name.

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

src/ui/window/plot.rs

@@ -365,6 +365,11 @@ pub struct PlotDialogState {
     /// sentinels.
     #[serde(skip)]
     pub printers: Vec<String>,
+    /// Why the system gave no printer list (a stopped spooler, a failing
+    /// print service), shown under the printer row so an empty list is
+    /// never mistaken for "no printers".
+    #[serde(skip)]
+    pub printers_error: Option<String>,
     /// Name of the system default printer, shown next to the default entry.
     #[serde(skip)]
     pub default_printer: Option<String>,
@@ -452,6 +457,7 @@ impl Default for PlotDialogState {
     fn default() -> Self {
         Self {
             printers: Vec::new(),
+            printers_error: None,
             default_printer: None,
             printer_media: None,
             custom_papers: Vec::new(),
@@ -1090,6 +1096,13 @@ pub fn view_window(
     // editor itself unfolds there when Properties… is pressed.
     let printer_note: Element<'_, Message> = match (&s.printer_editor, &s.printer) {
         (Some(draft), _) => printer_options_editor(draft, width),
+        (None, _) if s.printers_error.is_some() => {
+            let error = s.printers_error.clone().unwrap_or_default();
+            text(crate::tf!("Could not list printers: {error}"))
+                .size(10)
+                .style(muted_style)
+                .into()
+        }
         (None, Some(printer)) if s.driver_options.get(printer).is_some_and(|o| !o.is_empty()) => {
             let count = s.driver_options[printer].len();
             text(crate::tf!("{count} driver option(s) set for this printer."))