Synology Mail Station uses Roundcube for web based mail. Although Synology has a CardDAV Server now, the Roundcube they ship with Mail Station has no plugin for actually using it.
This installable package for Synology DSM adds the CardDAV interface to Roundcube, allowing usage of the contacts from your CardDAV server.
This package is based on the excellent "RoundCubeMail CardDAV" plugin of Benjamin "blind-coder" Schieder, as published on: https://github.com/blind-coder/rcmcarddav.