app: wire web export, preview, and print flows through the plot UI

- plot dialog: the web build defaults to 'Save to PDF file...' (its only
  output choice); native dropdowns and defaults are untouched.
- on_plot_dlg_commit forces to_file on wasm so a stale page setup can no
  longer route the web build into the (unreachable) printer path; both
  preview branches now go through preview_page_pdf, which is identical to
  the old inline temp-PDF + OS-viewer flow on desktop.
- Print ribbon / PRINTALL on wasm render the PDF and open it in a tab
  instead of erroring; PrintAllPdf picks a virtual path and downloads.
- pdf_plot_options maps the dialog's Quality dropdown into PdfPlotOptions.

src/app/update/mod.rs

@@ -8032,11 +8032,10 @@ impl OpenCADStudio {
                 }
                 #[cfg(target_arch = "wasm32")]
                 {
-                    let _ = stem;
-                    self.command_line.push_error(
-                        crate::t!("PDF export is not available in the web version.").as_ref(),
-                    );
-                    Task::none()
+                    Task::perform(
+                        crate::io::pdf_export::pick_pdf_path_owned(stem),
+                        Message::PrintAllPdfPath,
+                    )
                 }
             }
             Message::PrintAllPdfPath(None) => Task::none(),