Feature: code contents in long compressed link

### Feature request

By putting the written content in the link in an anchor, the frontend could even live-update the URL while you're typing, so that there isn't even a separate save button to press. The Lean playground does this: https://live.lean-lang.org/#codez=CYUwZgBJBcEHIEMAuFoF4IEYBQQ

The issue names no files or tests. Start by locating the frontend's current link and save behavior, then review the linked Lean playground for the URL-anchor approach. Done means the written content is encoded in the link and the URL can update while typing without a separate save button.

Written by the indexing model from the issue text.