fix(myqbz): context menus — z-index, Remove/queue wiring, sidebar + album->collection

- Detail row ⋯ menu (and the hero overflow + filter popups) were PopupWindows
  with no explicit height, so they collapsed and the rows below painted over them.
  Gave each an explicit height (+ close-on-click) like every other app menu, so
  they paint above the rows.
- Row menu Remove now actually removes: it was a no-op ("remove" wasn't a RowMode)
  -> wired to remove the item (repo::remove_item by position, reload). Add-to-queue
  was also silently broken (emitted "play-later", unparsed) -> now "add-to-queue".
- Sidebar playlist right-click menu: added "Add to Mixtape/Collection" (opens the
  picker with the playlist payload).
- Album -> Collection: an album can now be added to ANY collection kind incl.
  artist_collection (removed the ArtistCollection exclusion in load_rows; Tauri
  allows adding an album to an artist discography).

Offline in the row menu is net-new (Tauri's collection row menu has none; it would
be a batch resolve-then-cache) and is NOT added here. Backend crates untouched.
cargo check -p qbz-slint green.

cc

crates/qbz-slint/ui/shell/Sidebar.slint

@@ -119,6 +119,7 @@ component SidebarRow inherits Rectangle {
     callback edit-folder(string);
     callback hide-playlist(string);
     callback hide-folder(string);
+    callback add-to-mixtape(string);
     // Fires on row hover enter/leave so the sidebar can reveal the scrollbar
     // while the pointer is anywhere over the scrollable list.
     callback hover-changed(bool);
@@ -404,6 +405,14 @@ component SidebarRow inherits Rectangle {
                     menu.close();
                 }
             }
+            if !root.is-folder: ContextMenuItem {
+                icon: @image-url("../assets/icons/cassette-tape.svg");
+                label: @tr("Add to Mixtape/Collection");
+                clicked => {
+                    root.add-to-mixtape(root.entry.id);
+                    menu.close();
+                }
+            }
             // Divider before the move-to-folder section.
             if !root.is-folder: Rectangle {
                 height: 7px;
@@ -867,6 +876,9 @@ export component Sidebar inherits Rectangle {
                     hide-folder(id) => {
                         SidebarActions.hide-folder(id);
                     }
+                    add-to-mixtape(id) => {
+                        SidebarActions.add-to-mixtape(id);
+                    }
                     hover-changed(h) => {
                         pl-sb.hover-rows = h ? pl-sb.hover-rows + 1 : Math.max(0, pl-sb.hover-rows - 1);
                     }