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>

src/app/automation.rs

@@ -541,6 +541,56 @@ mod tests {
         );
     }
 
+    #[test]
+    fn exportpdf_with_explicit_path_writes_pdf_without_dialog() {
+        // EXPORTPDF <path> must export straight to the given file — no save
+        // dialog — and confirm on the command line. This is the dialog-free
+        // export path added for #369 (Export to PDF silently did nothing when
+        // the native save dialog could not open).
+        let mut app = OpenCADStudio::new_for_test();
+        app.automation_op(r#"{"op":"new"}"#);
+        app.automation_op(r#"{"op":"run","cmd":"LINE 0,0 100,50"}"#);
+
+        let path = std::env::temp_dir().join(format!("ocs_test_369_{}.pdf", std::process::id()));
+        let _ = std::fs::remove_file(&path);
+        let start = app.command_line.history.len();
+        let _ = app.run_command_line(&format!("EXPORTPDF {}", path.display()));
+        let out: String = app.command_line.history[start..]
+            .iter()
+            .map(|e| e.text.as_str())
+            .collect::<Vec<_>>()
+            .join("\n");
+        assert!(out.contains("Exported"), "no export confirmation: {out:?}");
+
+        let bytes = std::fs::read(&path).expect("EXPORTPDF <path> should write the file");
+        assert!(bytes.starts_with(b"%PDF"), "output is not a PDF");
+        assert!(bytes.len() > 200, "suspiciously small PDF: {}", bytes.len());
+        let _ = std::fs::remove_file(&path);
+    }
+
+    #[test]
+    fn cancelled_pdf_save_dialog_reports_on_command_line() {
+        // The save dialog resolving to None (cancelled, or it never opened —
+        // e.g. a broken XDG portal) must leave a visible message, not silence.
+        // Regression for #369.
+        use crate::app::Message;
+        let mut app = OpenCADStudio::new_for_test();
+        app.automation_op(r#"{"op":"new"}"#);
+        for msg in [Message::PlotExportPath(None), Message::PlotWindowExportPath(None)] {
+            let start = app.command_line.history.len();
+            let _ = app.update(msg);
+            let out: String = app.command_line.history[start..]
+                .iter()
+                .map(|e| e.text.as_str())
+                .collect::<Vec<_>>()
+                .join("\n");
+            assert!(
+                out.contains("EXPORTPDF"),
+                "dialog-None must mention the EXPORTPDF fallback: {out:?}"
+            );
+        }
+    }
+
     #[test]
     fn save_then_open_round_trips() {
         let mut app = OpenCADStudio::new_for_test();