M3.96: Color Themes panel with a local theme library

Window → Color Themes: the Create tab makes a five-colour theme on a
harmony wheel (a harmony rule from the selected colour, colours moving
together; or Custom, each on its own), with a brightness slider, hex
field, current-colour picker and name; the My Themes tab lists saved
themes to edit, add to Swatches as a colour group, or delete. Themes are
kept in the preferences (Prefs::color_themes; no online service).

Commands: colorTheme.list, colorTheme.save {colors | color + rule, name?,
replace?}, colorTheme.delete, colorTheme.addToSwatches. Harmony::theme
and move_on_wheel live in the colour crate; the wheel is now a shared
widget (widgets::harmony_wheel) used by Recolor Artwork's Edit tab too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -7,6 +7,7 @@ mod buildcmds;
 pub mod clipboard;
 mod colorcmds;
 pub mod colormgmt;
+pub mod colortheme;
 mod create;
 pub(crate) mod distortcmds;
 mod docinfo;
@@ -208,6 +209,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(expand::specs());
         v.extend(attributes::specs());
         v.extend(newart::specs());
+        v.extend(colortheme::specs());
         v
     })
 }