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/shell/PlayerBar.slint

@@ -16,7 +16,7 @@ import { Radius } from "../foundation/radius.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { QbzSlider } from "../primitives/QbzSlider.slint";
 import { SeekBar } from "SeekBar.slint";
-import { NowPlayingState } from "../state.slint";
+import { NowPlayingState, ShellState, AppearanceState } from "../state.slint";
 
 component IconButton inherits Rectangle {
     in property <image> icon;
@@ -257,6 +257,24 @@ export component PlayerBar inherits Rectangle {
                     IconButton { icon: @image-url("../assets/icons/maximize-2.svg"); icon-size: 16px; }
                     IconButton { icon: @image-url("../assets/icons/audio-lines.svg"); icon-size: 16px; }
 
+                    // Lyrics + queue panel toggles (moved here from the header).
+                    IconButton {
+                        icon: @image-url("../assets/icons/captions.svg");
+                        icon-size: 16px;
+                        active: ShellState.right-panel == 2;
+                        clicked => {
+                            ShellState.right-panel = ShellState.right-panel == 2 ? 0 : 2;
+                        }
+                    }
+                    IconButton {
+                        icon: @image-url("../assets/icons/list-ordered.svg");
+                        icon-size: 16px;
+                        active: ShellState.right-panel == 1;
+                        clicked => {
+                            ShellState.right-panel = ShellState.right-panel == 1 ? 0 : 1;
+                        }
+                    }
+
                     Rectangle { width: 6px; }
 
                     IconButton {
@@ -281,14 +299,15 @@ export component PlayerBar inherits Rectangle {
                             }
                         }
                     }
-                    IconButton {
+                    // Volume +/- steppers — opt-in (Settings > Appearance).
+                    if AppearanceState.show-volume-steppers: IconButton {
                         icon: @image-url("../assets/icons/minus.svg");
                         icon-size: 15px;
                         clicked => {
                             NowPlayingState.set-volume(Math.max(0.0, NowPlayingState.volume - 0.05));
                         }
                     }
-                    IconButton {
+                    if AppearanceState.show-volume-steppers: IconButton {
                         icon: @image-url("../assets/icons/plus.svg");
                         icon-size: 15px;
                         clicked => {