bug: /login?error=auth_error is produced by the callback but never shown by the login form

[shauryagangrade]

Problem

When OAuth fails, src/app/auth/callback/route.ts:48 redirects to /login?error=auth_error. But src/app/login/login-form.tsx never reads an error search param — users land on a pristine login form with no message explaining why they're back.

Fix

Read error from useSearchParams() and render a dismissible alert ("Sign-in failed or was cancelled — please try again"). Map known codes to friendly copy.

Acceptance criteria

• Failed OAuth returns show a visible error message on /login