feat(ui): folders slide open and closed, rows below follow

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

@@ -3,6 +3,7 @@
              xmlns:vm="using:ClaudeDo.Ui.ViewModels.Islands"
              xmlns:converters="using:ClaudeDo.Ui.Converters"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
+             xmlns:ctl="using:ClaudeDo.Ui.Views.Controls"
              x:Class="ClaudeDo.Ui.Views.Islands.ListsIslandView"
              x:DataType="vm:ListsIslandViewModel">
   <!-- Implicit, type-matched templates: the sidebar's top level is one mixed ItemsControl over
@@ -162,13 +163,14 @@
 
           <!-- 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 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"/>
-            <ItemsControl ItemsSource="{Binding Children}" Margin="14,0,0,0"/>
-          </Panel>
+          <ctl:ExpandPanel IsExpanded="{Binding !IsCollapsed}">
+            <Panel>
+              <Border Width="1" HorizontalAlignment="Left" Margin="8,0,0,4"
+                      Background="{Binding Color, Converter={x:Static converters:HexBrushConverter.Instance}, ConverterParameter=LineBrush}"
+                      IsHitTestVisible="False"/>
+              <ItemsControl ItemsSource="{Binding Children}" Margin="14,0,0,0"/>
+            </Panel>
+          </ctl:ExpandPanel>
         </StackPanel>
 
         <Border Grid.Row="2" Height="2" VerticalAlignment="Center" Margin="4,0"