Decide how to handle retracted studies in explorer app

## Context

The explorer app displays replication study data. Some of the original studies may have been retracted after publication.

## Questions to resolve

- Should retracted studies be displayed differently (e.g., with a warning indicator)?
- Should retracted studies be filterable/excludable from results?
- Should retraction information be shown in the study details?
- What data source should be used to identify retracted studies? (Note: `data/retraction-watch.csv` exists in the repo)

## Related

The annotator app already has retraction-checking functionality (`annotator/js/retraction-checker.js`).