feat(ui): animate folder expand in the sidebar

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -867,6 +867,19 @@
     <Setter Property="BorderBrush"     Value="Transparent" />
   </Style>
   <!-- Folder tree rows sit tighter than the smart lists — a deep tree gets long fast. -->
+  <!-- Folder expand: children drop down from under the header. Collapse stays instant. -->
+  <Style Selector="Panel.folder-children">
+    <Setter Property="RenderTransformOrigin" Value="50%,0%" />
+    <Setter Property="RenderTransform" Value="scaleY(1)" />
+  </Style>
+  <Style Selector="Panel.folder-children.animate[IsVisible=True]">
+    <Style.Animations>
+      <Animation Duration="0:0:0.16" Easing="CubicEaseOut" FillMode="Forward">
+        <KeyFrame Cue="0%">  <Setter Property="ScaleTransform.ScaleY" Value="0.4"/><Setter Property="Opacity" Value="0"/></KeyFrame>
+        <KeyFrame Cue="100%"><Setter Property="ScaleTransform.ScaleY" Value="1"/><Setter Property="Opacity" Value="1"/></KeyFrame>
+      </Animation>
+    </Style.Animations>
+  </Style>
   <Style Selector="Border.list-item.tree-row">
     <Setter Property="Padding" Value="4,4,10,4" />
   </Style>

src/ClaudeDo.Ui/ViewModels/Islands/ListFolderNavItemViewModel.cs

@@ -18,6 +18,8 @@ public sealed partial class ListFolderNavItemViewModel : ViewModelBase
     [ObservableProperty] private string _iconKey = "Folder";
     /// <summary>Hex colour for name + guide line; null = theme default.</summary>
     [ObservableProperty] private string? _color;
+    /// <summary>Set on the first user toggle so the expand animation skips folders that load already open.</summary>
+    [ObservableProperty] private bool _animateExpand;
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(ShowCount))]
     private bool _isCollapsed;

src/ClaudeDo.Ui/ViewModels/Islands/ListsIslandViewModel.cs

@@ -730,6 +730,7 @@ public sealed partial class ListsIslandViewModel : ViewModelBase, IDisposable
     private void ToggleFolder(ListFolderNavItemViewModel? folder)
     {
         if (folder is null) return;
+        folder.AnimateExpand = true;
         folder.IsCollapsed = !folder.IsCollapsed;
         if (_folderCollapseCts.TryGetValue(folder.Id, out var previous)) previous.Cancel();
         var cts = new CancellationTokenSource();

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml

@@ -162,7 +162,8 @@
 
           <!-- Guide line drops from under the chevron's centre (tree-row padding 4 + half the 9px
                glyph); children start past it so their highlight never touches the line. -->
-          <Panel IsVisible="{Binding !IsCollapsed}">
+          <Panel Classes="folder-children" Classes.animate="{Binding AnimateExpand}"
+                 IsVisible="{Binding !IsCollapsed}">
             <Border Width="1" HorizontalAlignment="Left" Margin="8,0,0,4"
                     Background="{Binding Color, Converter={x:Static converters:HexBrushConverter.Instance}, ConverterParameter=LineBrush}"
                     IsHitTestVisible="False"/>