fix(slint): flyouts/idle, header nav layout, toasts, settings, search

Flyouts & idle (main effort):
- Header dropdowns now glue under the tab: open-menu resets from-sidebar +
  title (a stale sidebar flyout state was flinging the panel far away).
- Idle-close now pauses while the cursor rests on the TRIGGER too (full
  header tabs gained the trigger-hovered logic the others already had).
- Dropped the dropdown caret (saves width on small screens).

Header nav layout:
- Section nav moved INTO the left-controls cluster, after the three sacred
  buttons, so it never overlaps them. Full text tabs when nav-in-sidebar off
  and sidebar not closed; compact icons when closed — mutually exclusive (no
  more duplicate/cluttered header).
- Search field gives up 60px (animated) when the header hosts the nav.

Settings made functional:
- Toast component mounted in AppShell (it was never rendered) + the in-app
  toasts toggle drives ToastState.enabled.
- "Show album artwork in Local Library track list" now drives the Tracks/
  Folders rows. Both rows un-flagged as unsupported.

Collapsible search (ExpandableSearch, app-wide):
- Smooth open AND close (width animates both ways; magnifier fades).
- Reliable auto-focus on open (deferred via a one-shot timer).

cc

crates/qbz-slint/ui/locallibrary/LocalLibraryView.slint

@@ -18,7 +18,7 @@
 
 import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
-import { LocalLibraryState, LocalLibraryActions, FolderNode, LocalArtistItem, AlphaJump, LibAlbumFilterState } from "../state.slint";
+import { LocalLibraryState, LocalLibraryActions, FolderNode, LocalArtistItem, AlphaJump, LibAlbumFilterState, AppearanceState } from "../state.slint";
 import { NavButtons } from "../shell/NavButtons.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { AlbumCollectionView } from "../discover/AlbumCollectionView.slint";
@@ -993,7 +993,7 @@ export component LocalLibraryView inherits Rectangle {
                                 TrackRow {
                                     track: track;
                                     index: i;
-                                    show-artwork: false;
+                                    show-artwork: AppearanceState.local-library-track-artwork;
                                     show-album: LocalLibraryState.tracks-group-mode != "album";
                                     show-favorite: false;
                                     show-download: false;
@@ -1309,7 +1309,7 @@ export component LocalLibraryView inherits Rectangle {
                                         for track[ti] in LocalLibraryState.folder-detail-tracks: TrackRow {
                                             track: track;
                                             index: ti;
-                                            show-artwork: false;
+                                            show-artwork: AppearanceState.local-library-track-artwork;
                                             show-album: true;
                                             show-favorite: false;
                                             show-download: false;