Apple Sign-In with Flutter and Supabase

If your Flutter app offers any third-party login on iOS — Google, Facebook, anything — Apple's App Review guideline 4.8 forces you to also offer Sign in with Apple. Skipping it is a guaranteed App Store rejection. That part is non-negotiable. The painful part is the setup: Apple Developer Console, Services IDs, .p8 keys, capabilities, entitlements, deep links — and most of it is only documented across three or four different Apple and Supabase pages that don't quite agree with each other. This i