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/update/mod.rs

@@ -3535,7 +3535,18 @@ impl OpenCADStudio {
                     Message::PlotExportPath,
                 )
             }
-            Message::PlotExportPath(None) => Task::none(),
+            // None means the save dialog was cancelled — or never opened at
+            // all (a broken XDG portal / missing zenity on Linux resolves to
+            // None too, and rfd only reports that through `log`, which is
+            // opt-in). Either way say something instead of silently doing
+            // nothing. (#369)
+            Message::PlotExportPath(None) => {
+                self.command_line.push_info(
+                    "PDF export canceled — no file chosen. \
+                     If no dialog appeared, use EXPORTPDF <path>.",
+                );
+                Task::none()
+            }
             Message::PlotExportPath(Some(path)) => self.on_plot_export_path_some(path),
 
             Message::PlotFormat(f) => {
@@ -3559,7 +3570,14 @@ impl OpenCADStudio {
                     Message::PlotWindowExportPath,
                 )
             }
-            Message::PlotWindowExportPath(None) => Task::none(),
+            // Same silent-None trap as PlotExportPath above. (#369)
+            Message::PlotWindowExportPath(None) => {
+                self.command_line.push_info(
+                    "PDF export canceled — no file chosen. \
+                     If no dialog appeared, use EXPORTPDF <path>.",
+                );
+                Task::none()
+            }
             Message::PlotWindowExportPath(Some(path)) => self.on_plot_window_export_path_some(path),
 
             // ── Print to system printer ───────────────────────────────────────