feat(locallibrary): nav scaffolding for the Local Library Slint view

Greenfield Local Library port, slice 1 — the view now mounts and navigates
end to end (empty placeholder body; per-tab data lands next):

- state.slint: `ContentView::local-library` + `LocalLibraryState`
  (active-tab / loading / load-error / per-category counts) +
  `LocalLibraryActions` (select-tab, open-settings). Re-exported from
  app.slint so the globals reach the Rust API.
- ui/locallibrary/LocalLibraryView.slint: header (title + stats + gear) +
  4-tab bar (Albums / Artists / Folders / Tracks) + placeholder body.
- shell: AppShell mounts the view on `local-library`; HeaderBar's
  Local Library menu items now carry routes (local-albums / -artists /
  -folders / -tracks) instead of being inert.
- nav.rs: `NavEntry::LocalLibrary { tab }` — per-tab history entry,
  mirroring Favorites / Discover (back/forward moves between tabs).
- src/local_library.rs: `LibTab` controller stub (route/tab-id parsing,
  round-trip tested).
- main.rs: apply_entry arm + `navigate_local_library` + header-route
  branch + in-view tab-bar (select-tab) and gear (open-settings ->
  Settings; management lives under Settings > Local Library) wiring.

qbz-slint cargo check green. Built blind — needs a runtime smoke test
(./scripts/slint-dev.sh): click the Local Library header menu, confirm the
view shows and the 4 tabs switch.

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, 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, CreatePlaylistState, CreatePlaylistActions, EditPlaylistState, EditPlaylistActions, CreateFolderState, CreateFolderActions, DragState, DragActions, PlaylistManagerState, PlaylistManagerActions, OfflineManagerState, OfflineManagerActions, LocalLibraryState, LocalLibraryActions, 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`.