Extract the GitHub release probe, download/relaunch state and the markdown
changelog renderer out of AppUpdater.tsx:
- utils/appUpdaterHelpers.ts isNewer, fmtBytes, pickAsset, types
- hooks/useAppUpdater.ts release probe + download/relaunch handlers
- components/appUpdater/Changelog.tsx
Pure code-move, no behaviour change. The AppShell call site is unchanged.