Generate image picker thumbnails with libvips in parallel (#6686)

Thumbnails were generated one ImageMagick process at a time. Queue the
missing ones and drain them across every core with vipsthumbnail, which
decodes and encodes faster and lets the per-process startup overlap.

Cold cache for a 92 wallpaper directory drops from 14.2s to 1.3s, and
the bundled theme previews from 2.0s to 0.26s.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

install/omarchy-base.packages

@@ -69,6 +69,7 @@ lazydocker
 lazygit
 less
 libsecret
+libvips
 libyaml
 libreoffice-fresh
 llvm