feat(ui): shared colour selector used across all style editors (#94)

Add a reusable color_selector widget (ui/color_select.rs): a main-colour
dropdown plus a "more colours" button that expands the full 255-entry ACI
palette — the same design the properties panel uses. Replace the flat ACI
swatch rows added earlier in the dimension, multileader and table style
editors with it, wired through each editor's existing colour edit message
(the chosen colour is encoded as an ACI string) plus a per-field
palette-open toggle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/mod.rs

@@ -4,6 +4,7 @@ pub const ROW_H: f32 = 26.0;
 
 pub mod about;
 pub mod app_menu;
+pub mod color_select;
 pub mod command_line;
 pub mod cycle_popup;
 pub mod dimstyle;