Frontend gains a feature-detected display path for two relationship
rails. Detail responses may now carry also_by_author and in_series.
When present, the detail page mounts Also-by-author followed by
In-series, with the current book highlighted inside the series strip.
When the backend omits them — which is the case today — the rails are
not rendered, so this lands as zero visible change until the backend
ships the data.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
