Expo + Supabase GitHub Auth Broke 3 Times — Here's the Fix

TL;DR: GitHub login in my Expo app broke three separate times. A deep link that went nowhere. A PKCE flow I wired backwards. A redirect URL with a typo. Each fix is copy-pasteable below. Total auth code is under a hundred lines. Steal it. Auth is the worst part of every app. Nobody downloads your app for the login screen. They tolerate it. Every...