feat: manage apfel updates in-app

[sfegette]

Summary

Currently release notes instruct users to run brew upgrade apfel manually. grapfel should detect when a newer apfel version is available and offer (or perform) the upgrade in-app.

Target behavior

• On launch (or periodically), check whether the installed apfel version matches the latest tap release
• If outdated, show an update prompt in the app (similar to Sparkle for grapfel itself)
• Invoke brew upgrade apfel programmatically on user confirmation
• Show progress and surface errors

Notes

• Coordinate with the auto-install flow from the companion issue — shared HomebrewInstaller surface
• Consider whether to bundle a minimum required apfel version and block usage if too old