Add last-resort/backup system for drawing save events #4421
Description
Had a few reports of folks loosing drawings after spending a lot of time editing (and not saving) either due to session timeouts or others.
Would be best to store image data to the browser before pushing to the server, for potential access/restore in the event something goes wrong.
Not yet sure what the UI would look like, or where it even fits in.
Maybe check and prompt to restore on diagrams.net boot, if old content found in store?