feat(ui): collapsible folders for the list sidebar

Folders render above the top-level lists, each with its own collapsible
children; tapping a folder header folds it instead of selecting it.
Lists move in by dragging onto a folder header (or onto a row inside it)
and out by dropping between top-level rows or via the context menu.

Creating a folder is a toggle in the New-list modal, not a third sidebar
button: it reduces the modal to name + icon and writes a folder row
instead of the list row the create flow pre-created.

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

@@ -5,6 +5,91 @@
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:Class="ClaudeDo.Ui.Views.Islands.ListsIslandView"
              x:DataType="vm:ListsIslandViewModel">
+  <UserControl.Resources>
+    <!-- One user-list row. Shared by the top-level list and by each folder's children, so the
+         row (context menu, drag handlers, drop indicators) exists once. -->
+    <DataTemplate x:Key="UserListRow" DataType="vm:ListNavItemViewModel">
+      <Grid RowDefinitions="Auto,Auto,Auto">
+
+        <!-- Above-row drop indicator -->
+        <Border Grid.Row="0" Height="2" VerticalAlignment="Center" Margin="4,0"
+                Background="{DynamicResource MossBrush}" CornerRadius="1"
+                IsVisible="{Binding DropHintAbove}"/>
+
+        <Border Grid.Row="1" Classes="list-item" Classes.active="{Binding IsActive}"
+                Classes.drop-target="{Binding IsTaskDropTarget}"
+                Tapped="OnItemTapped"
+                DragDrop.AllowDrop="True"
+                DragDrop.DragOver="OnListDragOver"
+                DragDrop.Drop="OnListDrop">
+          <Border.ContextMenu>
+            <ContextMenu>
+              <MenuItem Header="{loc:Tr lists.contextWorktrees}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenWorktreesOverviewCommand}"
+                        CommandParameter="{Binding}"/>
+              <Separator IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+              <MenuItem Header="{loc:Tr lists.contextOpenExplorer}"
+                        IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenInExplorerCommand}"
+                        CommandParameter="{Binding}"/>
+              <MenuItem Header="{loc:Tr lists.contextOpenFindings}"
+                        IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenFindingsCommand}"
+                        CommandParameter="{Binding}"/>
+              <MenuItem Header="{loc:Tr lists.contextOpenTerminal}"
+                        IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenInTerminalCommand}"
+                        CommandParameter="{Binding}"/>
+              <MenuItem Header="{loc:Tr lists.contextImportTickets}"
+                        IsVisible="{Binding HasTicketProject}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).ImportTicketsCommand}"
+                        CommandParameter="{Binding}"/>
+              <Separator/>
+              <MenuItem Header="{loc:Tr lists.contextRemoveFromFolder}"
+                        IsVisible="{Binding FolderId, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).RemoveFromFolderCommand}"
+                        CommandParameter="{Binding}"/>
+              <MenuItem Header="{loc:Tr lists.contextSettings}"
+                        Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenListSettingsCommand}"
+                        CommandParameter="{Binding}"/>
+            </ContextMenu>
+          </Border.ContextMenu>
+          <Grid ColumnDefinitions="20,*,Auto">
+            <!-- Left accent bar for active state -->
+            <Border Grid.Column="0" Grid.ColumnSpan="3"
+                    Background="Transparent"
+                    CornerRadius="8" IsHitTestVisible="False"
+                    IsVisible="{Binding IsActive}">
+              <Border Width="2" Height="16"
+                      Background="{DynamicResource AccentBrush}"
+                      CornerRadius="1"
+                      HorizontalAlignment="Left" VerticalAlignment="Center"
+                      Margin="-8,0,0,0"/>
+            </Border>
+            <!-- Icon (list-settings → Icon tab; "Folder" when unset) -->
+            <PathIcon Grid.Column="0" Classes="list-icon"
+                      Width="14" Height="14"
+                      Data="{Binding IconKey, Converter={StaticResource IconKey}}"
+                      HorizontalAlignment="Center" VerticalAlignment="Center"/>
+            <!-- Name -->
+            <TextBlock Grid.Column="1" Classes="list-label"
+                       Text="{Binding Name}"
+                       VerticalAlignment="Center" Margin="8,0"
+                       Foreground="{DynamicResource TextDimBrush}" FontSize="{StaticResource FontSizeBody}"/>
+            <!-- Count -->
+            <TextBlock Grid.Column="2" Classes="list-count"
+                       Text="{Binding Count}"/>
+          </Grid>
+        </Border>
+
+        <!-- Below-row drop indicator (last item only) -->
+        <Border Grid.Row="2" Height="2" VerticalAlignment="Center" Margin="4,0"
+                Background="{DynamicResource MossBrush}" CornerRadius="1"
+                IsVisible="{Binding DropHintBelow}"/>
+      </Grid>
+    </DataTemplate>
+  </UserControl.Resources>
+
   <DockPanel LastChildFill="True">
 
     <!-- ── Header ── -->
@@ -133,87 +218,61 @@
             <PathIcon Data="{StaticResource Icon.Folder}" Width="14" Height="14"/>
           </Button>
         </Grid>
-        <ItemsControl ItemsSource="{Binding UserLists}">
+        <!-- Folders first, each with its own (collapsible) child lists, then the top-level lists.
+             Folders are never selectable — tapping the header folds it. -->
+        <ItemsControl ItemsSource="{Binding Folders}">
           <ItemsControl.ItemTemplate>
-            <DataTemplate DataType="vm:ListNavItemViewModel">
-              <Grid RowDefinitions="Auto,Auto,Auto">
+            <DataTemplate DataType="vm:ListFolderNavItemViewModel">
+              <StackPanel>
+                <Border Classes="list-item"
+                        Classes.drop-target="{Binding IsDropTarget}"
+                        Tapped="OnFolderTapped"
+                        DragDrop.AllowDrop="True"
+                        DragDrop.DragOver="OnFolderDragOver"
+                        DragDrop.Drop="OnFolderDrop">
+                  <Border.ContextMenu>
+                    <ContextMenu>
+                      <MenuItem Header="{loc:Tr lists.contextFolderSettings}"
+                                Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenFolderSettingsCommand}"
+                                CommandParameter="{Binding}"/>
+                    </ContextMenu>
+                  </Border.ContextMenu>
+                  <Grid ColumnDefinitions="12,20,*,Auto">
+                    <PathIcon Grid.Column="0" Classes="list-icon"
+                              Width="9" Height="9"
+                              Data="{StaticResource Icon.ChevronRight}"
+                              IsVisible="{Binding IsCollapsed}"
+                              HorizontalAlignment="Left" VerticalAlignment="Center"/>
+                    <PathIcon Grid.Column="0" Classes="list-icon"
+                              Width="9" Height="9"
+                              Data="{StaticResource Icon.ChevronDown}"
+                              IsVisible="{Binding !IsCollapsed}"
+                              HorizontalAlignment="Left" VerticalAlignment="Center"/>
+                    <PathIcon Grid.Column="1" Classes="list-icon"
+                              Width="14" Height="14"
+                              Data="{Binding IconKey, Converter={StaticResource IconKey}}"
+                              HorizontalAlignment="Center" VerticalAlignment="Center"/>
+                    <TextBlock Grid.Column="2" Classes="list-label"
+                               Text="{Binding Name}"
+                               VerticalAlignment="Center" Margin="8,0"
+                               Foreground="{DynamicResource TextDimBrush}" FontSize="{StaticResource FontSizeBody}"/>
+                    <TextBlock Grid.Column="3" Classes="list-count"
+                               Text="{Binding Count}"/>
+                  </Grid>
+                </Border>
 
-                <!-- Above-row drop indicator -->
-                <Border Grid.Row="0" Height="2" VerticalAlignment="Center" Margin="4,0"
-                        Background="{DynamicResource MossBrush}" CornerRadius="1"
-                        IsVisible="{Binding DropHintAbove}"/>
-
-              <Border Grid.Row="1" Classes="list-item" Classes.active="{Binding IsActive}"
-                      Classes.drop-target="{Binding IsTaskDropTarget}"
-                      Tapped="OnItemTapped"
-                      DragDrop.AllowDrop="True"
-                      DragDrop.DragOver="OnListDragOver"
-                      DragDrop.Drop="OnListDrop">
-                <Border.ContextMenu>
-                  <ContextMenu>
-                    <MenuItem Header="{loc:Tr lists.contextWorktrees}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenWorktreesOverviewCommand}"
-                              CommandParameter="{Binding}"/>
-                    <Separator IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
-                    <MenuItem Header="{loc:Tr lists.contextOpenExplorer}"
-                              IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenInExplorerCommand}"
-                              CommandParameter="{Binding}"/>
-                    <MenuItem Header="{loc:Tr lists.contextOpenFindings}"
-                              IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenFindingsCommand}"
-                              CommandParameter="{Binding}"/>
-                    <MenuItem Header="{loc:Tr lists.contextOpenTerminal}"
-                              IsVisible="{Binding WorkingDir, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenInTerminalCommand}"
-                              CommandParameter="{Binding}"/>
-                    <MenuItem Header="{loc:Tr lists.contextImportTickets}"
-                              IsVisible="{Binding HasTicketProject}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).ImportTicketsCommand}"
-                              CommandParameter="{Binding}"/>
-                    <Separator/>
-                    <MenuItem Header="{loc:Tr lists.contextSettings}"
-                              Command="{Binding $parent[UserControl].((vm:ListsIslandViewModel)DataContext).OpenListSettingsCommand}"
-                              CommandParameter="{Binding}"/>
-                  </ContextMenu>
-                </Border.ContextMenu>
-                <Grid ColumnDefinitions="20,*,Auto">
-                  <!-- Left accent bar for active state -->
-                  <Border Grid.Column="0" Grid.ColumnSpan="3"
-                          Background="Transparent"
-                          CornerRadius="8" IsHitTestVisible="False"
-                          IsVisible="{Binding IsActive}">
-                    <Border Width="2" Height="16"
-                            Background="{DynamicResource AccentBrush}"
-                            CornerRadius="1"
-                            HorizontalAlignment="Left" VerticalAlignment="Center"
-                            Margin="-8,0,0,0"/>
-                  </Border>
-                  <!-- Icon (list-settings → Icon tab; "Folder" when unset) -->
-                  <PathIcon Grid.Column="0" Classes="list-icon"
-                            Width="14" Height="14"
-                            Data="{Binding IconKey, Converter={StaticResource IconKey}}"
-                            HorizontalAlignment="Center" VerticalAlignment="Center"/>
-                  <!-- Name -->
-                  <TextBlock Grid.Column="1" Classes="list-label"
-                             Text="{Binding Name}"
-                             VerticalAlignment="Center" Margin="8,0"
-                             Foreground="{DynamicResource TextDimBrush}" FontSize="{StaticResource FontSizeBody}"/>
-                  <!-- Count -->
-                  <TextBlock Grid.Column="2" Classes="list-count"
-                             Text="{Binding Count}"/>
-                </Grid>
-              </Border>
-
-                <!-- Below-row drop indicator (last item only) -->
-                <Border Grid.Row="2" Height="2" VerticalAlignment="Center" Margin="4,0"
-                        Background="{DynamicResource MossBrush}" CornerRadius="1"
-                        IsVisible="{Binding DropHintBelow}"/>
-              </Grid>
+                <ItemsControl ItemsSource="{Binding Lists}"
+                              IsVisible="{Binding !IsCollapsed}"
+                              Margin="14,0,0,0"
+                              ItemTemplate="{StaticResource UserListRow}"/>
+              </StackPanel>
             </DataTemplate>
           </ItemsControl.ItemTemplate>
         </ItemsControl>
 
+        <ItemsControl ItemsSource="{Binding RootLists}"
+                      ItemTemplate="{StaticResource UserListRow}"/>
+
       </StackPanel>
     </ScrollViewer>
   </DockPanel>

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

@@ -69,6 +69,60 @@ public partial class ListsIslandView : UserControl
         }
     }
 
+    private void OnFolderTapped(object? sender, RoutedEventArgs e)
+    {
+        if (sender is Border { DataContext: ListFolderNavItemViewModel folder }
+            && DataContext is ListsIslandViewModel vm)
+            vm.ToggleFolderCommand.Execute(folder);
+    }
+
+    // Dropping onto the folder *header* appends the list to that folder — the only route into a
+    // collapsed or empty folder, where there's no child row to drop next to.
+    private void OnFolderDragOver(object? sender, DragEventArgs e)
+    {
+        if (DataContext is not ListsIslandViewModel vm) { e.DragEffects = DragDropEffects.None; return; }
+        if ((!e.DataTransfer?.Contains(ListRowFormat) ?? true)
+            || sender is not Border { DataContext: ListFolderNavItemViewModel folder })
+        {
+            e.DragEffects = DragDropEffects.None;
+            vm.ClearDropHints();
+            return;
+        }
+
+        var sourceId = e.DataTransfer?.TryGetValue(ListRowFormat);
+        var source = sourceId is null ? null : vm.UserLists.FirstOrDefault(r => r.Id == sourceId);
+        if (source is null || source.FolderId == folder.Id)
+        {
+            e.DragEffects = DragDropEffects.None;
+            vm.ClearDropHints();
+            return;
+        }
+
+        vm.SetFolderDropTarget(folder);
+        e.DragEffects = DragDropEffects.Move;
+    }
+
+    private async void OnFolderDrop(object? sender, DragEventArgs e)
+    {
+        if (DataContext is not ListsIslandViewModel vm) return;
+        try
+        {
+            if (sender is not Border { DataContext: ListFolderNavItemViewModel folder }) return;
+
+            var sourceId = e.DataTransfer?.TryGetValue(ListRowFormat);
+            var source = sourceId is null ? null : vm.UserLists.FirstOrDefault(r => r.Id == sourceId);
+            if (source is null) return;
+
+            vm.ClearDropHints();
+            await vm.MoveToFolderAsync(source, folder);
+        }
+        catch
+        {
+            vm.ClearDropHints();
+            throw;
+        }
+    }
+
     private void OnListDragOver(object? sender, DragEventArgs e)
     {
         if (DataContext is not ListsIslandViewModel vm) { e.DragEffects = DragDropEffects.None; return; }
@@ -150,11 +204,14 @@ public partial class ListsIslandView : UserControl
         return null;
     }
 
+    // Next row *within the same container* — the row below the last list of a folder belongs to
+    // another folder (or to the top level), so it is not a "drop above" candidate here.
     private static ListNavItemViewModel? FindNextUserList(ListsIslandViewModel vm, ListNavItemViewModel row)
     {
-        var idx = vm.UserLists.IndexOf(row);
+        var container = vm.ContainerOf(row);
+        var idx = container.IndexOf(row);
         if (idx < 0) return null;
-        return idx + 1 < vm.UserLists.Count ? vm.UserLists[idx + 1] : null;
+        return idx + 1 < container.Count ? container[idx + 1] : null;
     }
 
 }