feat(statusbar): customizable pills with coordinates and clean screen

Add a customization menu (far-right handle) that lists every status-bar
pill with a check mark; toggling a row shows or hides that pill, and the
choice persists across sessions in the config dir.

New pills:
- Coordinates: live cursor readout (re-adds the model-space world offset
  so it shows true drawing coordinates), updated on every move rather
  than only while a command is active.
- Clean Screen: hide the ribbon and side panels for a full canvas.

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

src/ui/mod.rs

@@ -21,6 +21,8 @@ pub mod scale_popup;
 pub mod shortcuts;
 pub mod snap_popup;
 pub mod statusbar;
+pub mod statusbar_config;
+pub mod statusbar_menu;
 pub mod tablestyle;
 pub mod textstyle;