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/develop.rs

@@ -2648,9 +2648,7 @@ impl DevelopSession {
         // Refuse a category this run did not produce rather than creating a
         // layer that renders empty: an empty mask looks like a broken
         // adjustment, where a button that does nothing at least says so.
-        if seg.category_mask(name).is_none() {
-            return None;
-        }
+        seg.category_mask(name)?;
 
         let id = self.graph.masks().next_id();
         let mut layer = MaskLayer::new(

ui/dr-ui/src/library_ui.rs

@@ -2255,7 +2255,7 @@ fn load_window(window: &AppWindow, ctl: &Rc<LibraryController>) {
     //
     // The trash is never reorderable. It is a view of what was deleted, ordered
     // by when, and it is not a collection at all.
-    window.set_library_manual_order(
+    window.set_library_reorderable(
         !trash
             && scope.is_some_and(|c| {
                 dr_catalog::collections::orders_manually(catalog.connection(), c).unwrap_or(false)

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"))]
     {