[FE-58] Build freight analytics heatmap by region

[mftee]

Overview

Build a choropleth/heatmap visualization showing freight volume and average cost by region or country. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/FreightHeatmap.tsx
• World or regional map with countries/regions colored by shipment volume
• Color scale: light = low volume, dark = high volume
• Tooltip on hover: region name, shipment count, average cost
• Toggle between: Shipment Volume, Average Cost, On-Time Rate
• Color scale legend shown below the map
• Demo page at frontend/sandbox/analytics/page.tsx renders with mock data