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.

(cherry picked from commit 892d693d2d)

src/app/mod.rs

@@ -3712,6 +3712,8 @@ pub enum Message {
     /// Send current layout to the system printer (via lp / lpr).
     PrintToPrinter,
     /// Callback from the async printer job.
+    /// Web never constructs it — printing opens the PDF preview instead.
+    #[cfg_attr(target_arch = "wasm32", allow(dead_code))]
     PrintResult(Result<String, String>),
     /// Open the full Plot / Print dialog (seeds state from the layout).
     PlotDialogOpen,