v0.16.1: clearer source errors and Drive freshness reporting

Fixed

• Missing or malformed source responses now report a lookup failure instead of an empty notebook. Studio preserves source lookup errors, including timeouts. Notes remain separate from sources.
• Drive freshness checks distinguish unknown status from confirmed stale sources, and sync reports skipped sources.

The source diagnostics were prompted by #397. We have not confirmed that this release resolves the reporter’s underlying issue.

Credits

Thanks to @insane66613 for #396 and @akperkins for reporting #397.