fix(plot): never fail silently on PDF export; add EXPORTPDF <path> (#369)

Export to PDF could end in complete silence: the flow hands off to an
async rfd save dialog, and when that future resolves to None the app
mapped it straight to Task::none(). None does not only mean the user
cancelled — on Linux the XDG portal SaveFile call can fail outright
(rfd then tries a zenity fallback, which may not be installed) and the
future still just resolves to None. rfd reports those failures only
through the `log` crate, and logging is opt-in (--log / RUST_LOG), so
the reporter saw exactly what #369 describes: no dialog, no error, no
console output, no file.

- EXPORT / EXPORTPDF now accept an optional path argument that exports
  straight to the file with no dialog — both the workaround for systems
  where the native save dialog cannot open and a headless-testable
  entry point. A relative path lands next to the drawing; a missing
  extension gets .pdf.
- The dialog-None arms (PlotExportPath / PlotWindowExportPath) now push
  a command-line message pointing at EXPORTPDF <path> instead of doing
  nothing.
- The success message prints the full output path, not just the file
  name, so users can find the PDF.

Verified end to end on the real binary via --serve: LINE + CIRCLE then
EXPORTPDF <path> writes a valid PDF with the expected geometry. Both
new tests fail on the previous code.

Fixes #369

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

COMMANDS.md

@@ -303,8 +303,8 @@ Status of every standard CAD command in Open CAD Studio:
 | `QSAVE` | — | Quick save | ✅ |
 | `SAVEALL` | — | Save all open drawings | ✅ |
 | `PLOT` | — | Print / plot | ✅ |
-| `EXPORT` | — | Export | ✅ |
-| `EXPORTPDF` | — | Export to PDF | ✅ |
+| `EXPORT [path]` | — | Export to PDF (save dialog, or straight to `path`) | ✅ |
+| `EXPORTPDF [path]` | — | Export to PDF (save dialog, or straight to `path`) | ✅ |
 | `PAGESETUP` | — | Page setup | ✅ |
 | `PLOTSTYLE` | — | Plot style | ✅ |
 | `PURGE` | PU | Purge unused items | ✅ |