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/commands/mod.rs

@@ -534,6 +534,19 @@ inventory::submit!(crate::command::CommandRegistration {
         "SKPOLY",
         "SKTOLERANCE",
         "CLIPROMPTLINES",
+        "COLORTHEME",
+        "SELECTIONAREA",
+        "SELECTIONAREAOPACITY",
+        "SELECTIONEFFECT",
+        "SELECTIONEFFECTCOLOR",
+        "WINDOWSAREACOLOR",
+        "WINDOWAREACOLOR",
+        "CROSSINGAREACOLOR",
+        "SELECTIONPREVIEW",
+        "GRIPSIZE",
+        "GRIPCOLOR",
+        "GRIPHOT",
+        "GRIPHOVER",
         // Reset selected entities' overrides to follow their layer.
         "SETBYLAYER",
         // Remove duplicate objects; set drawing base point; audit integrity;