Guest Login Implemented

Add a discoverable guest-browsing entry point on login/signup

Guest access to /library worked (middleware creates an anonymous
session transparently), but there was no way for a visitor to know it
existed: / redirects straight to /login, and the login/signup forms
had no link out. Adds a "Browse the library without an account" link
to AuthForm.tsx (shared by both pages) pointing at /library — the
middleware handles the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>