Show small thumbnails of recently completed ArchiveResult content in the
progress header. The thumbnail strip appears below the stats bar and shows
the last 20 successfully archived items with embeddable content (screenshots,
favicons, DOM snapshots, etc.).
Features:
- API returns recent_thumbnails with embed paths for succeeded results
- Thumbnails display with plugin-specific icons as fallback
- New thumbnails animate in with a pop effect
- Clicking a thumbnail navigates to the snapshot admin page
- Horizontal scrollable strip with custom scrollbar styling
