feat(model-space): theme-aware model space canvas and CAD selection/grip sysvars

Model space appearance now follows the active UI theme instead of a
hardcoded dark background. Canvas mode is user-selectable (Match Theme /
Classic CAD Dark / Custom RGB), with matching controls for the paper
sheet and desk surround backgrounds and grid opacity.
Implements industry-standard CAD system variables, all settable via
command name, SETVAR, and the Options dialog:
- COLORTHEME (0/1 dark/light UI scheme, preserves custom palettes)
- COLORSCHEME (interactive named-theme selection)
- BACKGROUND (THEME/CLASSIC/DEFAULT/DESK presets, RGB triplets)
- SELECTIONAREA, SELECTIONAREAOPACITY (translucent marquee fill)
- WINDOWSAREACOLOR/WINDOWAREACOLOR, CROSSINGAREACOLOR (ACI or theme)
- SELECTIONEFFECT, SELECTIONEFFECTCOLOR (GPU highlight pass, driven by
  selection_generation invalidation — no retessellation)
- SELECTIONPREVIEW (bit 0 = idle rollover, bit 1 = in-command hover)
- GRIPSIZE, GRIPCOLOR, GRIPHOT, GRIPHOVER (grip overlay styling)

src/app/view/modal.rs

@@ -174,8 +174,8 @@ impl OpenCADStudio {
             }
             super::super::ModalKind::Options => sized_flow(
                 ex,
-                520,
-                500,
+                540,
+                560,
                 |flow| {
                     crate::ui::window::options::view_window(
                         &self.default_save_format,
@@ -190,6 +190,10 @@ impl OpenCADStudio {
                         self.crosshair_color,
                         &self.crosshair_color_input,
                         self.lineweight_display_scale,
+                        &self.model_space,
+                        &self.model_bg_input,
+                        &self.paper_bg_input,
+                        &self.desk_bg_input,
                         flow,
                     )
                 },