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.
