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/settings/AppearanceSettings.slint

@@ -14,7 +14,7 @@ import { Theme } from "../foundation/semantic-colors.slint";
 import { ThemeState } from "../foundation/theme-state.slint";
 import { Typography } from "../foundation/typography.slint";
 import { Radius } from "../foundation/radius.slint";
-import { AppearanceState, SidebarState, ShellState } from "../state.slint";
+import { AppearanceState, SidebarState, ShellState, ToastState } from "../state.slint";
 import { QbzToggle } from "../primitives/QbzToggle.slint";
 import { QbzSelect } from "../primitives/QbzSelect.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
@@ -312,7 +312,6 @@ export component AppearanceSettings inherits VerticalLayout {
         }
     }
     SettingRow {
-        unsupported: true;
         label: @tr("Show album artwork in Local Library track list");
         description: @tr("Display the album cover thumbnail next to each track in the Tracks and Folders views. Off by default — large libraries pay a per-row image-decode cost.");
         QbzToggle {
@@ -334,12 +333,12 @@ export component AppearanceSettings inherits VerticalLayout {
     GroupHeader { text: @tr("NOTIFICATIONS"); }
 
     SettingRow {
-        unsupported: true;
         label: @tr("In-app toasts notifications");
         QbzToggle {
             checked: AppearanceState.in-app-toasts;
             toggled(v) => {
                 AppearanceState.in-app-toasts = v;
+                ToastState.enabled = v;
                 AppearanceState.appearance-bool("in-app-toasts", v);
             }
         }