use localStorage to backup work

We should save edits in localStorage as people make them. This way if the tab crashes or the user accidentally closes their window/browser we can prompt them to load their last state.
This should be optional, not everyone opening a new blockbuilder tab will want their old code.

We should also load the code in paused execution, this will act as a temporary solution if the crash was caused by a runaway for/while loop (assuming we don't address those issues more fundamentally soon).

The issue names no files or tests; first locate the browser editor’s edit-state and execution entry points. Trace how edits are stored and how code starts running, then define the optional recovery prompt and paused-load behavior, with coverage for those flows as the completion check.

Written by the indexing model from the issue text.