With the sidebar fully closed there was no way to reach playlists. Add a
playlists button to the closed-sidebar header cluster that opens a flyout
over SidebarState's entries (folders + playlists), six rows visible with
the rest scrolling, plus an always-visible footer linking to the Playlist
Manager. Rendered at the AppShell level (SidebarPlaylistsPopup +
SidebarPlaylistsPopupState); header button needs no absolute-position
correction (not inside a Flickable).
cc
