v1.8.0

Minor Changes

• #19 a5fd650 Thanks @ryuhzk! - A theme's preview image is published beside its package. When a manifest
  declares preview, build copies that asset out of the packed theme -- the
  bytes as packed, so the optimised WebP -- into dist/previews/<id>.<ext>,
  with the extension (webp, png or jpg) chosen by inspecting the bytes,
  and the index entry gains preview: "dist/previews/<id>.<ext>". A gallery
  can show the picture without downloading the package.

  dist/previews/ mirrors the index: a theme that stops declaring a preview,
  or whose source is gone, loses its file, and build removes anything the
  index does not name. check reports a missing, stale or stray preview file
  and names muqun-theme build as the fix. list --json carries previewUrl
  beside url when the index came from a URL and the entry has a preview.

• #19 d9d4cc5 Thanks @ryuhzk! - init scaffolds a placeholder preview cover (assets/preview.png, 1024x640)
  and names it in preview, so a fresh theme already has what galleries show.
  check --sources --require-preview refuses a source without one; the themes
  repository's CI uses it.

npm: https://www.npmjs.com/package/@osuki-dev/muqun-theme/v/1.8.0