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>

locales/ru-RU/opencadstudio.ftl

@@ -5100,6 +5100,12 @@ plot =
     .system-default-printer = Системный принтер по умолчанию
     .what-to-plot = Что спланировать
     .x-mm = X (мм)
+    .x-in = X (дюймы)
+    .y-in = Y (дюймы)
+    .hide-paperspace-objects = Скрыть объекты пространства листа
+    .save-changes-to-layout = Сохранить изменения в листе
+    .printer-does-not-support-sheet-switched-to-its-default-sheet-default-paperupdate-1 = __ocs_fmt_0__ не поддерживает __ocs_fmt_1__; выбран формат по умолчанию __ocs_fmt_2__ (PAPERUPDATE = 1).
+    .printer-does-not-support-sheet-the-sheet-is-kept-paperupdate-0 = __ocs_fmt_0__ не поддерживает __ocs_fmt_1__; формат сохранён (PAPERUPDATE = 0).
     .y-mm = Y (мм)
 
 manage =