[FE-59] Implement shipment CO2 emissions calculator

[mftee]

Overview

Build a component that estimates the carbon footprint of a shipment based on transport mode, distance, and cargo weight. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/EmissionsCalculator.tsx
• Inputs: transport mode (Road, Sea, Air, Rail), distance (km), cargo weight (kg)
• Calculates CO2 estimate using standard emission factors per mode
• Displays result in kg CO2 with a color indicator (green < 100kg, yellow 100-500kg, red > 500kg)
• Shows equivalent trees needed to offset the emissions
• Results update in real time as inputs change
• Demo page at frontend/sandbox/page.tsx renders the calculator