feat: interactive feature showcase on the signup panel

Replaces the static quote in the onboarding layout's left panel with
an auto-rotating carousel of product highlights (Deploy / Manage /
Scale, 3 slides x 4 items), shown on the cloud signup page only.

- OnboardingLayout gets an optional leftPanel override, default
  quote untouched everywhere else (login, invitation, password reset).
- Auto-advances every 6s, pauses on hover/focus, respects
  prefers-reduced-motion, manual dot + arrow navigation.
- Copy pulled from the website's existing first-features.tsx section
  to stay consistent with approved marketing copy.