Replace layer thumbnails with Vello renders

この issue には説明がありません。

The issue names no files, tests, or entry points. Start by locating the existing layer-thumbnail rendering path and how Vello is integrated, then determine the scope of replacing the thumbnails with Vello renders. Done means layer thumbnails are rendered through Vello and the relevant behavior is covered by the project's existing checks.

索引モデルが issue の本文から書いたものです。