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/file.rs

@@ -1119,7 +1119,10 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 }
     }
 
-    pub(super) fn on_plot_export_path_some(&mut self, path: std::path::PathBuf) -> Task<Message> {
+    pub(in crate::app) fn on_plot_export_path_some(
+        &mut self,
+        path: std::path::PathBuf,
+    ) -> Task<Message> {
                 let i = self.active_tab;
                 let scene = &self.tabs[i].scene;
                 let wires = scene.entity_wires();
@@ -1222,10 +1225,12 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                     &path,
                     self.active_plot_style.as_ref(),
                 ) {
-                    Ok(()) => self.command_line.push_info(&format!(
-                        "Exported: {}",
-                        path.file_name().unwrap_or_default().to_string_lossy()
-                    )),
+                    // Full path, not just the file name — when the export was
+                    // driven by EXPORTPDF <path> the user needs to see where
+                    // the file actually landed. (#369)
+                    Ok(()) => self
+                        .command_line
+                        .push_info(&format!("Exported: {}", path.display())),
                     Err(e) => self.command_line.push_error(&format!("Export failed: {e}")),
                 }
                 Task::none()