Cloud documents with short links

Move between synchronized documents without full page reloads, using compact Letter.md links.

Letter.md became a cloud-connected workspace without giving up the speed of local editing.

What changed

Cloud documents received stable, compact Letter.md URLs.
The library can switch between documents without reloading the whole app.
New untitled documents open immediately, then synchronize when the cloud session is ready.
Local editor recovery and real-time collaboration converge on the same Markdown document.

Anonymous local editing still works without an account. Sign-in is only needed for cloud sync, sharing, and collaboration.