[FE-61] Create shipment rate card component

[mftee]

Overview

Build a rate card component that displays a carrier's pricing structure for different cargo types and routes. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/RateCard.tsx
• Shows: base rate per kg, minimum charge, fuel surcharge %, handling fee
• Rate table for different cargo types (General, Refrigerated, Hazmat)
• Route-specific rates section (if applicable)
• Valid from/to date shown
• Request Custom Quote button
• Accepts rates: RateStructure as props
• Demo page at frontend/sandbox/carriers/[id]/rates/page.tsx renders with mock data

[debojeetmitra]

Hi @mftee,
I would like to work on this issue. Please assign it to me if possible. Thanks!