feat(slint): split sidebar toggle from nav buttons, animate side panels

NavButtons now renders the collapse toggle and history buttons as two
spaced pills instead of separating them with a bar. Queue and Lyrics
sidebars animate their own width open/closed (160ms ease-in-out) so
they slide in like the main sidebar instead of appearing abruptly.

cc

crates/qbz-slint/ui/shell/QueueSidebar.slint

@@ -110,7 +110,11 @@ component QueueRow inherits Rectangle {
 }
 
 export component QueueSidebar inherits Rectangle {
-    width: 320px;
+    // Always present; the width animates 0 <-> 320 so the panel slides
+    // in smoothly, matching the left sidebar's collapse animation.
+    width: ShellState.right-panel == 1 ? 320px : 0px;
+    clip: true;
+    animate width { duration: 160ms; easing: ease-in-out; }
     background: Theme.surface-card;
 
     VerticalLayout {