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/commands/mod.rs

@@ -12,6 +12,7 @@ mod fileops;
 mod inquiry;
 mod layerprops;
 mod layers;
+mod plotvars;
 mod styleprops;
 mod view;
 
@@ -263,6 +264,9 @@ impl OpenCADStudio {
         if let Some(t) = self.dispatch_layerprops(cmd, i) {
             return Some(t);
         }
+        if let Some(t) = self.dispatch_plotvars(cmd, i) {
+            return Some(t);
+        }
         if let Some(t) = self.dispatch_styleprops(cmd, i) {
             return Some(t);
         }
@@ -555,6 +559,14 @@ inventory::submit!(crate::command::CommandRegistration {
         "DELOBJ",
         "PLINEGEN",
         "PSLTSCALE",
+        // Plot preferences and the current-layout variables (commands/plotvars.rs).
+        "PLOTOFFSET",
+        "PAPERUPDATE",
+        "PLOTROTMODE",
+        "PLOTTRANSPARENCYOVERRIDE",
+        "BACKGROUNDPLOT",
+        "CTAB",
+        "TILEMODE",
         "DISPSILH",
         "WORLDVIEW",
         "LIMCHECK",