Reconcile what four branches each built separately

Three seams, found by the first compile after the merge.

Two branches implemented 'can this scope be reordered' independently:
one from the collections model, one from the catalog through
orders_manually, on every re-read and excluding the trash. The second is
the better answer and is what survives; it only needed to set the
property app.slint declares.

Two lints from scene-mask-ui, which was merged mid-flight and had never
been through -D warnings: an is_none check spelled out where clippy wants
?, and a return in a cfg block's tail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/segmentation.rs

@@ -590,7 +590,7 @@ fn scene_categories(
 fn load_scene_model() -> Result<dr_segment::SceneModel, String> {
     #[cfg(feature = "scene-model")]
     {
-        return dr_segment::SceneModel::embedded().map_err(|e| e.to_string());
+        dr_segment::SceneModel::embedded().map_err(|e| e.to_string())
     }
     #[cfg(not(feature = "scene-model"))]
     {