Move the library grid's cells and selection onto their own Slint global

AppWindow carried the grid's loaded window of cells, the keyboard cursor,
drag and drop, the held-row long-press state, columns and cell size, the
scroll and viewport bookkeeping, the photo roll's pick and centre-request,
and the local-only/reorder/collection-filing gestures that act on a
selection, as properties and callbacks on the root component. That state now
lives in the `Library` global declared in library.slint, next to the structs
(LibraryCell, TimelineBar, KeywordRow, PersonChip) it and the grid's other
components already share; Rust reaches it through
window.global::<Library>() instead of window.set_/get_/on_/invoke_ on the
root, the same change collections.slint's `Collections` global made for the
sidebar.

library-visible stays on AppWindow: it is computed from active-page and
active-view, the shell's own routing state, which a global cannot read.
Everything else still prefixed library- — the timeline, the filter bar,
ratings and flags, keywording, and the routes and status lines — stays on
the window for now and moves in the commits that follow.

ui/dr-ui/src/library_ui/offline.rs

@@ -13,7 +13,7 @@ use dr_catalog::Catalog;
 use slint::ComponentHandle;
 
 use crate::library;
-use crate::AppWindow;
+use crate::{AppWindow, Library};
 
 use super::controller::{stop, LibraryController};
 
@@ -499,7 +499,9 @@ fn refresh_local_count(window: &AppWindow, ctl: &Rc<LibraryController>) {
     let Some(catalog) = borrow.as_ref() else {
         return;
     };
-    window.set_library_local_count(library::local_original_count(catalog).unwrap_or(0) as i32);
+    window
+        .global::<Library>()
+        .set_library_local_count(library::local_original_count(catalog).unwrap_or(0) as i32);
 }
 
 /// TRACES: FR-NC-6a