[FE-60] Build carrier onboarding multi-step form

[mftee]

Overview

Build a multi-step onboarding form for new carriers to register their business, fleet, and service areas. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/carrier/onboarding/page.tsx
• Step 1: Business details (company name, registration number, country)
• Step 2: Fleet information (vehicle types, capacity, count)
• Step 3: Service areas (origin regions, destination regions, cargo types accepted)
• Step 4: Document upload (business license, insurance certificate)
• Step 5: Review and submit
• Progress bar at top showing current step
• Form state persists between steps
• Uses Zod + React Hook Form for validation