feat(statusbar): drawing-units pill with INSUNITS picker

Add a Units pill that shows the drawing's current INSUNITS value and
opens a picker to change it (Unitless, mm, cm, m, km, in, ft, mi, yd).
Toggleable from the status-bar customization menu like the other pills.

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

src/ui/mod.rs

@@ -25,6 +25,7 @@ pub mod statusbar_config;
 pub mod statusbar_menu;
 pub mod tablestyle;
 pub mod textstyle;
+pub mod units_popup;
 
 pub use app_menu::AppMenu;
 pub use command_line::CommandLine;