fix(qt): Ctrl+A and Escape reach the offline manager, and Escape leaves mix select mode

The AppShell hotkey seam duck-types selectAll, exitMultiSelectMode and
multiSelectOn on the mounted view. The offline manager exposed none of
them, so Ctrl+A did nothing there although the Slint ticked every track,
and MixView lacked multiSelectOn, so Escape never left its select mode. The
seam's comment, which still listed Artist, Playlist, Mix and Label as views
without multi-select, now names the views that implement it.

cc

crates/qbz-qt/qml/shell/AppShell.qml

@@ -244,10 +244,11 @@ Rectangle {
     // in-view QML JS (library_bulk.rs:8: "select-all / clear never reach
     // Rust"), so the QbzShell signals route to the mounted view's
     // duck-typed interface: selectAll() / exitMultiSelectMode() /
-    // multiSelectOn. Implemented by LibraryView, LocalLibraryView and
-    // LocalAlbumView (MyQbzDetailView is EXIT-ONLY — its selection lives in
-    // Rust with no select-all arm); views without multi-select (Artist,
-    // Playlist, Mix, Label, Offline) match nothing — PARITY-DEBT (K4).
+    // multiSelectOn. Implemented by every view with a multi-select list:
+    // Library, Local Library, Local Album, Album, Artist, Playlist, Mix,
+    // Label and the Offline manager. MyQbzDetailView is EXIT-ONLY — its
+    // selection lives in Rust and its bar has no select-all arm (the Slint
+    // never had one there either).
     Connections {
         target: QbzShell
         function onSelectAllRequested() {

crates/qbz-qt/qml/views/MixView.qml

@@ -124,7 +124,10 @@ Rectangle {
         if (action !== "add-to-playlist" && action !== "add-to-mixtape")
             root.selected = ({})
     }
-    // Ctrl+A / Escape hotkey seam (AppShell duck-types these).
+    // Ctrl+A / Escape hotkey seam (AppShell duck-types these). Without
+    // `multiSelectOn` the Escape stack never saw this view in select mode, so
+    // Escape did not leave it here while it did on every other track list.
+    readonly property bool multiSelectOn: root.multiSelect
     function selectAll() {
         if (!root.multiSelect) root.setMultiSelect(true)
         root.bulkAction("select-all")

crates/qbz-qt/qml/views/OfflineManagerView.qml

@@ -119,6 +119,18 @@ Rectangle {
                                         mods === undefined ? Qt.NoModifier : mods)
     }
 
+    // Ctrl+A / Escape hotkey seam (AppShell duck-types these). The selection
+    // here is always on (no select mode), so Ctrl+A ticks every track — as
+    // the Slint's select_all_active_surface did for this view — and Escape
+    // drops a non-empty selection.
+    readonly property bool multiSelectOn: Object.keys(root.selected).length > 0
+    function selectAll() {
+        root.bulkAction("select-all")
+    }
+    function exitMultiSelectMode() {
+        root.bulkAction("clear")
+    }
+
     function bulkAction(id) {
         if (id === "clear") {
             root.selected = ({})