feat(slint): classic now-playing bar layout

Add the second of the four switchable now-playing bar layouts plus the
4-layout architecture behind it.

ShellState.npb-mode (0=New, 1=Classic) drives mode-conditional columns in
PlayerBar; the transport cluster is extracted into TransportControls.slint and
IconButton/PlayButton into primitives/BarControls (PlayButton gains a `circle`
prop) so both layouts share them without duplicating the popups. The choice
persists via UiPrefs.npb_mode ("new"/"classic") — restored on startup, written
on the "Now Playing view" menu select.

Classic = the Tauri arrangement: transport LEFT (plain play glyph, leading
track-info, inline add-to-playlist + favorite-with-state), glass song card
CENTRE, actions + volume RIGHT. SongCard.glass is a contained card capped at a
fixed 520px; the audio stamp is ABSOLUTELY pinned to the card's right edge
(cover 56 | song info that truncates with ellipsis | stamp 140) so it stays
flush right regardless of the text layout. Track info fires from both the
title click and the transport (i). Classic uses the full-size audio stamp; the
backend LED reads PIPEWIRE.

cc

crates/qbz-slint/ui/app.slint

@@ -18,7 +18,7 @@ import { AppShell } from "shell/AppShell.slint";
 import { Toast } from "primitives/Toast.slint";
 
 // Re-export the state globals so the Rust layer can populate them.
-export { HomeState, HomeActions, DiscoverBrowseState, DiscoverBrowseActions, ForYouState, MixState, GenreFilterState, GenreFilterActions, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions, PlaylistPickerState, PlaylistPickerActions, PlaylistState, PlaylistActions, SidebarState, SidebarActions, SidebarFolderPopupState, CreatePlaylistState, CreatePlaylistActions, EditPlaylistState, EditPlaylistActions, CreateFolderState, CreateFolderActions, DragState, DragActions, PlaylistManagerState, PlaylistManagerActions, OfflineManagerState, OfflineManagerActions, LocalLibraryState, LocalLibraryActions, LibraryFoldersState, LibFolderEditState, LibraryManageActions, LibraryScanState, LibAlbumFilterState, LocalAlbumState, LocalAlbumActions, TagEditorState, TagEditorActions, FolderEditState, FolderEditActions, ToastState, TextUtil } from "state.slint";
+export { HomeState, HomeActions, DiscoverBrowseState, DiscoverBrowseActions, ForYouState, MixState, GenreFilterState, GenreFilterActions, AlbumState, ArtistState, NavState, ShellState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions, PlaylistPickerState, PlaylistPickerActions, PlaylistState, PlaylistActions, SidebarState, SidebarActions, SidebarFolderPopupState, CreatePlaylistState, CreatePlaylistActions, EditPlaylistState, EditPlaylistActions, CreateFolderState, CreateFolderActions, DragState, DragActions, PlaylistManagerState, PlaylistManagerActions, OfflineManagerState, OfflineManagerActions, LocalLibraryState, LocalLibraryActions, LibraryFoldersState, LibFolderEditState, LibraryManageActions, LibraryScanState, LibAlbumFilterState, LocalAlbumState, LocalAlbumActions, TagEditorState, TagEditorActions, FolderEditState, FolderEditActions, ToastState, TextUtil } from "state.slint";
 
 // Which top-level screen is shown. The app starts on `splash` while it
 // restores a saved session, then resolves to `shell` or `login`.