[FE-65] Create shipment feedback and rating form

[mftee]

Overview

Build a post-delivery feedback form where shippers rate their experience with a carrier after a shipment is completed. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/ShipmentFeedback.tsx
• Star rating (1-5) for: Overall Experience, Communication, Timeliness, Cargo Condition
• Optional comment textarea (max 500 chars with live counter)
• Would you use this carrier again? Yes/No toggle
• Submit button disabled until all star ratings are filled
• Success state after submission
• Accepts shipmentId, carrierId, onSuccess callback as props
• Demo page at frontend/sandbox/shipments/[id]/feedback/page.tsx renders the form