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/ViewModels/Islands/ListFolderNavItemViewModel.cs

@@ -0,0 +1,23 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui.ViewModels.Islands;
+
+/// <summary>
+/// A sidebar folder row. Not a <see cref="ListNavItemViewModel"/> and not a
+/// <see cref="ListKind"/> — a folder holds no tasks and is never selectable, so tapping it
+/// toggles <see cref="IsCollapsed"/> instead of changing the selection.
+/// </summary>
+public sealed partial class ListFolderNavItemViewModel : ViewModelBase
+{
+    public required string Id { get; init; }
+    [ObservableProperty] private string _name = "";
+    [ObservableProperty] private string _iconKey = "Folder";
+    [ObservableProperty] private bool _isCollapsed;
+    // Sum of the child lists' counts — a collapsed folder still shows what's inside it.
+    [ObservableProperty] private int _count;
+    // Set while a dragged list row hovers over this folder header (move-into-folder target).
+    [ObservableProperty] private bool _isDropTarget;
+
+    public ObservableCollection<ListNavItemViewModel> Lists { get; } = new();
+}