Thin Quart blueprint that delegates to GalleryService. Cursor-based scroll
returns images + a date_groups summary so the SPA can render year-month
headers without re-grouping client-side. Timeline returns year-month
buckets for the sidebar jump nav. Jump returns a cursor positioned at a
specific year-month so the gallery can scroll to historical periods.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>