CheckForUpdate now also resolves the release's downloadable asset (portable
.exe, or a .zip to unpack). DownloadAndApplyUpdate streams it with progress
events (update:progress 0-100), swaps it in for the running exe (rename the
current one to .old — allowed for a running image on Windows — move the new one
into place, roll back on failure), then relaunches with --post-update and quits.
main: a --post-update relaunch retries the single-instance mutex for ~20 s so
the fresh process waits for the old one to exit and free it, then clears the
.old exe left behind. Falls back to opening the release page when a release has
no auto-download asset.
rouggy