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/app/commands/mod.rs

@@ -712,6 +712,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "PERF",
         "PERSP",
         "PLOT",
+        "PRINTERS",
         "PLOTSTYLE",
         "PLOTSTYLEEDITOR",
         "PLOTSTYLEPANEL",