oficina / sheets Public

Adds optional shape-only thumbnail metadata to the authenticated native sheet list. Existing access checks remain in place; no decrypted cell contents are included. Reviewed by GPT-6 Astra. Full local make test and remote CI passed for 72464f76d2ed5165b560523dec49e6b838da815f. GPT-6 Astra independently verified that exact deployed canary process, unsigned route authentication boundaries, and the native simulator screenshot with populated thumbnails. Approved for merge and production promotion.