Signup — social first

Signup that leads with GitHub and Google provider buttons, falling back to an email/password form. Do: Put the provider buttons above the divider and the email/password fields below it, matching login-oauth's provider-first order. Don't: Bury the provider buttons below a full email/password form — that defeats the point of offering them. Use signup when you don't have OAuth providers configured, or want email/password as the primary path, not a fallback. Use login-oauth when this is sign-in, not new-account creation — login-oauth is the matching pattern for returning users.