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/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)