Auto-update only checked at launch, so users with auto-updates off (or who
just want it now) had no way to update from the app. Add an on-demand flow:
checkForUpdatesNow/downloadUpdateNow/quitAndInstallUpdate in auto-updater
(works regardless of the opt-in; not-packaged dev surfaces 'unsupported'),
forwarded to the renderer as a live UpdateStatus over updates:status. The
Updates section gains a Check for updates button and an Update button that
downloads then installs (Restart & install), with checking/available/
downloading%/downloaded/error status. Refs #2207.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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