[FE-56] Build cargo type selector component

[mftee]

Overview

Build a visual cargo type selector that lets shippers categorize their freight when creating a shipment. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/CargoTypeSelector.tsx
• Types: General Cargo, Refrigerated, Hazardous Materials, Oversized/Heavy Lift, Liquid Bulk, Dry Bulk, Vehicles
• Each type shown as a card with icon, label, and brief description
• Selected type highlighted with a colored border
• Selecting Hazardous Materials shows an additional UN number input field
• Calls onSelect(cargoType) callback
• Demo page at frontend/sandbox/page.tsx renders all types