- 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.