feat(plot): page-setup units, custom scale, hide-paperspace, save-to-layout and the plot sysvars

Plot dialog:
- "Units" (millimetres / inches) is the page setup's own unit: the offsets
  and the custom scale are shown in it, the stored ratio converts with it
  so the plotted result and the paper space's unit stay the same
- "Custom: [paper] mm = [drawing] units" spells the picked scale in that
  unit and accepts a typed ratio, which selects the list's name for it or
  becomes a ratio entry of its own
- "Hide paperspace objects" is the page setup's plot-hidden flag; it is no
  longer derived from the "Hidden Line" shade mode
- "Save changes to layout" (paper layouts, on by default) writes the dialog
  into the layout when a plot is sent; previews never touch it
- offsets are labelled in the page-setup unit and kept in millimetres in
  the file

System variables (typeable, through SETVAR, listed by SETVAR ?):
- PLOTOFFSET, PAPERUPDATE, PLOTROTMODE, PLOTTRANSPARENCYOVERRIDE and
  BACKGROUNDPLOT live with the persisted plot preferences: the plot origin
  counts from the printable corner unless PLOTOFFSET says the paper edge;
  a printer that does not list the sheet either warns or switches to its
  default sheet; transparency can be forced on or off; BACKGROUNDPLOT bit 1
  is the dialog's background switch and bit 2 drives batch plots
- CTAB and TILEMODE read and switch the current layout

Tests cover every new control and variable; six new strings in all 21
locales; the Plot dialog grows to 940×690 for the two extra rows.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/view/modal.rs

@@ -67,7 +67,7 @@ impl OpenCADStudio {
         sized_flow(
             extra,
             940,
-            620,
+            690,
             |flow| {
                 crate::ui::window::plot::view_window(
                     &self.plot_dialog,