Checks the GitHub releases for newer desktop-v tags once per launch
and every 24h (15s timeout, no identifying headers, offline = silent
retry). A dismissible banner and a Settings About row surface the
newer version and link the release page; dismissal persists per
version so each release nags at most once. No auto-install: unsigned
builds cannot, by platform constraint; the checker only notifies.
App 364/364 (+24), typechecks + build green.
