feat(statusbar): selection filtering by object type (FILTER pill)

Add a per-type selection filter to the scene: the FILTER pill opens a
checklist of the entity types in the layout, and unchecking a type
excludes it from interactive picking (click, box and lasso selection all
honour it). The pill lights up while any type is filtered out.

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

src/ui/mod.rs

@@ -19,6 +19,7 @@ pub mod plotstyle;
 pub mod properties;
 pub mod ribbon;
 pub mod scale_popup;
+pub mod selection_filter_popup;
 pub mod shortcuts;
 pub mod snap_popup;
 pub mod statusbar;