feat: show a page tree in the sidebar with drag to move and nest

[AmrMsCLL]

Notes step N4.

• The sidebar gets a Pages tree under the areas: every space expands to its pages and boards, and each page expands to the ones inside it. Which branches are open is remembered on the device.
• Hovering a row shows + to add a page inside it (or at the top of a space).
• Drag to move: drop on the top or bottom edge of a row to put the page before or after it, or on the middle to put it inside (not inside a board). Dropping on a space puts it at the top level. Moves stay within the space and use the existing PATCH /notes/:id (parentId, position); moving a page inside its own child is refused by the server as before.
• GET /me/notes without kind now returns pages and boards together (up to 500) for the tree.
• The tree is hidden while the sidebar is collapsed.

Checked in the browser (local stack): the Personal space expands to its pages and boards, with the board inside Menu test shown under it. Tests: new tree case in test/notes.spec.tsx (expand, nest by dragging, add inside) plus the specs that render the shell; the combined listing is in test/integration/notes.spec.ts (CI).