[FE-62] Build shipment consolidation planner UI

[mftee]

Overview

Build a UI that lets shippers combine multiple small shipments into one consolidated freight booking to reduce costs. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/shipments/consolidate/page.tsx
• List of eligible shipments (same destination region) with checkboxes
• Selected shipments show combined weight, volume, and estimated savings
• Incompatible cargo type warning (e.g. cannot mix Hazmat with General)
• Confirm Consolidation button creates a new consolidated shipment
• Max 10 shipments per consolidation
• Uses mock data if no API connected