electron-builder now publishes latest-linux.yml / latest.yml and embeds
app-update.yml plus package-type, so electron-updater runs pacman -U or
dpkg -i through pkexec for packages and updates the per-user NSIS install
without elevation. The app checks only when asked (menu item), asks before
downloading and before installing, and J621_UPDATE_URL overrides the feed
for tests or forks.
deploy/push_desktop.sh builds and publishes the artifacts to
deploy/data/desktop, which the frontend nginx mounts read-only at
/desktop/. Verified detection and up-to-date handling against a local feed
with the packaged Arch build.
