feat(slint): library/sidebar UX overhaul + theme switching + fixes

Shell & navigation:
- Three-state sidebar (open/mini/closed); collapse cycles via header button.
- Section nav as right-flyouts (sidebar) + compact header nav (closed) +
  full header tabs (nav-in-sidebar off), via a shared HeaderMenuOverlay.
- Collapsed-folder right-flyout popup (max 4 + scrollbar), Rust-populated.
- Thin auto-hiding sidebar scrollbar pinned to the edge (Flickable, drag-safe).
- Slack-style bezel: rounded inset content panel over surface-card chrome.
- Header restructure: nav buttons, window-centered search, app menu.

Themes & components:
- Live theme switching: Dark / OLED Black / Tokyo Night / System (std-widgets).
- SegmentedTabBar (Discovery-style) with count badges for Library + Local Library.
- MP3 quality-badge tier (format-first classification) across badges.
- QbzSelect (Group/Sort) trimmed 20px.

Behavior fixes:
- Artist + Local Library Folder cover art now reach the rendered (visible/
  grouped) models, not just the source set.
- Favorites Playlists/Labels random buttons.
- Playlist track click plays from the clicked track in visible order.
- Dropdown idle-close pauses while the cursor rests on the menu.

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, 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 } from "state.slint";
+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 } 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`.