feat: give boards and pages their own places in the sidebar

[AmrMsCLL]

Boards and pages were only reachable from a space's page list, so the whiteboard shipped in #76 was effectively hidden.

• Sidebar (and the phone's More page) gains Pages and Boards. Each lists every page or board across your spaces, newest edit first, with the space and when it changed.
• New board / New page creates one straight away in your only writable space, or asks which space when there are several, then opens it.
• New endpoint GET /me/notes?kind=board|note (templates left out, at most 200).

Checked in the browser on the local stack: Boards lists the existing board, and New board opens a fresh canvas. Tests: test/notes.spec.tsx (new Boards case), lint; the endpoint is covered in test/integration/notes.spec.ts (CI).