Image documents and the Asset library

Keep reusable images as workspace Assets and create photo-first documents that remain portable Markdown.

Letter.md now treats media as reusable workspace content instead of one-off attachments.

What changed

Create image documents with a focused photo-first editor.
Upload, browse, and reuse images from a shared Asset library.
See which documents reference an Asset before removing it.
Organize Assets into folders and select several images to start a new document.
Store image documents as ordinary Markdown plus original Assets for portability.

Assets belong to the workspace, so the same file can be used safely in more than one document.