Publish a site on letter.pub

Turn a Letter.md workspace into a public site with a hosted letter.pub address and explicit snapshots.

Letter.md can now publish documents as a coherent site instead of a collection of unrelated share links.

What changed

The first publication creates a site for the workspace.
Every site receives a stable hosted address on letter.pub.
Site settings keep the name, introduction, appearance, and public index together.
Each document publishes as an explicit read-only snapshot.
Public pages include an archive, feed, and sitemap where applicable.

Editing a document never changes the public version silently; publishing an update remains a deliberate action.