Style the writing-first sign-in panel and publish stepper
The SignInPanel's inner elements were unstyled (default input + grey
buttons), and the panel/menu cards used rgba()/Canvas colours that wash
out on the dark canvas. Mirror the editor's LoginForm (login.css) — display
title, muted lede, framed input, sun primary button, outline cancel, accent
signup link — and reframe the cards + account-pill menu on the paper-raised
surface with token borders/shadow so they read in light and dark.