Blocknote is missing unsaved content detection

Reproduction steps

1. edit a page with BlockNote
2. simulate a network error (e.g., block the save page url for the current backend)
3. try to leave the page (clicking on close, navigating to another page from the navigation tree...)

Expected

A warning message is raised, warning the user that unsaved content will be lost.

Actual

The user is not informed of the potential loss of content.

Note

See CRISTAL-218 for the equivalent issue for Tiptap.