Error: Invalid call to browser.storage.local.set(). Exceeded storage quota.

## Error

```
Invalid call to browser.storage.local.set(). Exceeded storage quota.
```

## Tracking Details
- **Project**: exercism-js
- **Sentry**: [Error: Invalid call to browser.storage.local.set(). Exceeded storage quota.](https://thalamus-ai.sentry.io/issues/145096162/)
- **Culprit**: https://exercism.org/dashboard
- **First seen:** Sep 5, 2026

Start with the linked Sentry report and the dashboard entry point identified as the culprit. Trace the browser.storage.local.set() calls in exercism-js and determine which stored data exceeds the quota. Reproduce the failure under the affected storage conditions, then verify that the dashboard no longer triggers this error.

Written by the indexing model from the issue text.