Form — Multi-step onboarding

Create form — multi-step onboarding.

Spec: 3-step wizard with progress bar and per-step validation.

Build with:
- Tailwind 4 + shadcn/ui primitives
- Fully responsive (stacks on mobile)
- Dark mode via next-themes
- Accessible: labels, aria where needed, keyboard nav
- TypeScript props

Return a single self-contained React component. Include mock data inline so it renders standalone.