Share sign-in form styles between dashboard and editor
The signed-out LoginForm renders on both /editor and /dashboard, but its
`login__*` rules lived only in editor-chrome.css (imported by editor.astro).
The dashboard never imported them, so the same form appeared unstyled there.
Extract the form rules into a shared src/styles/login.css and import it from
both pages, so the sign-in form looks identical wherever a signed-out visitor
lands. The editor-specific `.studio__login`/`.studio__error` rules stay in
editor-chrome.css.