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/AppShell.slint

@@ -108,10 +108,11 @@ export component AppShell inherits Rectangle {
                 }
             }
 
-            // Right-side panel — a layout sibling of the content area,
-            // so it shrinks the content rather than overlaying it.
-            if ShellState.right-panel == 1: QueueSidebar { }
-            if ShellState.right-panel == 2: LyricsSidebar { }
+            // Right-side panels — layout siblings of the content area, so
+            // they shrink the content rather than overlaying it. Always
+            // present; each animates its own width open/closed.
+            QueueSidebar { }
+            LyricsSidebar { }
         }
 
         PlayerBar {

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

@@ -37,7 +37,11 @@ component CloseButton inherits Rectangle {
 }
 
 export component LyricsSidebar 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 == 2 ? 320px : 0px;
+    clip: true;
+    animate width { duration: 160ms; easing: ease-in-out; }
     background: Theme.surface-card;
 
     VerticalLayout {

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

@@ -1,9 +1,9 @@
-// Browser-like back / forward navigation pair.
+// Sidebar-collapse toggle + browser-like back / forward navigation pair.
 //
-// Same visual format as the Discover tab pill group (an elevated rounded
-// container). Used inline beside the Discover tabs and in place of the
-// per-page Back button. Both buttons drive the NavState global, so the
-// Rust nav history records every move.
+// Two separate elevated pills with a gap between them: the sidebar
+// toggle stands on its own, the [<] [>] history pair is its own group.
+// Used inline beside the Discover tabs and in place of the per-page
+// Back button.
 
 import { Theme } from "../foundation/semantic-colors.slint";
 import { NavState, ShellState } from "../state.slint";
@@ -39,40 +39,50 @@ component NavBtn inherits Rectangle {
     }
 }
 
-export component NavButtons inherits Rectangle {
+// One elevated rounded pill wrapping its NavBtn children.
+component NavPill inherits Rectangle {
     background: Theme.surface-elevated;
     border-radius: 6px;
-    width: btn-row.preferred-width;
     height: 34px;
+    @children
+}
 
-    btn-row := HorizontalLayout {
-        padding: 3px;
-        spacing: 4px;
-        // Leading button: collapses / expands the left sidebar. Lives
-        // here so it appears just left of the [<] [>] pair in every view.
-        NavBtn {
-            icon: @image-url("../assets/icons/panel-left.svg");
-            clicked => {
-                ShellState.sidebar-collapsed = !ShellState.sidebar-collapsed;
+export component NavButtons inherits HorizontalLayout {
+    spacing: 8px;
+
+    // Sidebar collapse toggle — its own pill.
+    NavPill {
+        width: collapse-row.preferred-width;
+        collapse-row := HorizontalLayout {
+            padding: 3px;
+            NavBtn {
+                icon: @image-url("../assets/icons/panel-left.svg");
+                clicked => {
+                    ShellState.sidebar-collapsed = !ShellState.sidebar-collapsed;
+                }
             }
         }
-        // 1px separator between the collapse button and the history pair.
-        Rectangle {
-            width: 1px;
-            background: Theme.border-subtle;
-        }
-        NavBtn {
-            icon: @image-url("../assets/icons/chevron-left.svg");
-            enabled: NavState.can-back;
-            clicked => {
-                NavState.request-back();
+    }
+
+    // Back / forward history pair — its own pill.
+    NavPill {
+        width: history-row.preferred-width;
+        history-row := HorizontalLayout {
+            padding: 3px;
+            spacing: 4px;
+            NavBtn {
+                icon: @image-url("../assets/icons/chevron-left.svg");
+                enabled: NavState.can-back;
+                clicked => {
+                    NavState.request-back();
+                }
             }
-        }
-        NavBtn {
-            icon: @image-url("../assets/icons/chevron-right.svg");
-            enabled: NavState.can-forward;
-            clicked => {
-                NavState.request-forward();
+            NavBtn {
+                icon: @image-url("../assets/icons/chevron-right.svg");
+                enabled: NavState.can-forward;
+                clicked => {
+                    NavState.request-forward();
+                }
             }
         }
     }

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 {