Log in an existing user by exchanging an Auth Code issued during the PKCE flow.
supabase.auth.exchangeCodeForSession('34e770dd-9ff9-416c-87fa-43b31d7ef225')
const flowId = requestUrl.searchParams.get('sb_flow_id')
supabase.auth.exchangeCodeForSession(code, flowId ? { flowId } : undefined)