GitHub/Google buttons on the login page pointed at providers that may
not be configured in Supabase, dead-ending in an error. Add
NEXT_PUBLIC_OAUTH_GITHUB / NEXT_PUBLIC_OAUTH_GOOGLE (opt-in, "true" to
show); a provider button renders only when enabled and the whole social
block is hidden when neither is. Pure resolveOAuthProviders helper is
unit-tested; flags documented in .env.example and DEPLOYMENT.md.
