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/ListNavItemViewModel.cs

@@ -26,4 +26,6 @@ public sealed partial class ListNavItemViewModel : ViewModelBase
     [ObservableProperty] private bool _isTaskDropTarget;
     // Settable (not init) so a list-settings icon change refreshes the row in place.
     [ObservableProperty] private string? _iconKey;
+    // The sidebar folder this row sits in (ListEntity.FolderId); null = top level.
+    [ObservableProperty] private string? _folderId;
 }