[sheet] warn if using stale data

As an author, I would like to be notified if I am working with stale data so that I do not clobber updates made while I have my sheet open.

### Potential solution
1. Upon open, cache the original response JSON.
2. Poll every 10s (while tab is active) to ensure JSON matches the original response.
3. If the data has changed, pop a toast stating the data is out of date.

### Other ideas
We could do this check on save to eliminate the polling noise. The only challenge here is what do we do with the data we want to save? It gets messy unless we have some sort of diff (plus merge tools) we can provide. Doing this every 10s would mean you do not get very far with edits before you see the toast.

No files, tests, or entry points are named. Start by tracing the sheet's open and save flows and how the original response JSON is available, then clarify whether stale data is checked by polling or on save. Done should include an agreed notification behavior and a defined approach for handling edits after the data changes.

Written by the indexing model from the issue text.