load_config synchronously parsed every recent DWG's embedded preview
on the main thread before the first frame, so the Start page appeared
seconds late on DWG-heavy recents. refresh_recent_thumbs now decodes
the missing previews on a background thread and delivers them via
RecentThumbsLoaded; the page opens immediately and thumbnails pop in
as they resolve. The web build (no threads, no filesystem previews)
skips the fetch.
Co-Authored-By: Claude Fable 5 <[email protected]>