[FE-64] Build platform activity feed for admin

[mftee]

Overview

Build a real-time activity feed for admins showing recent platform events: new registrations, shipment status changes, disputes filed, and payments processed. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/AdminActivityFeed.tsx
• Event types with distinct icons: User Registered, Shipment Created, Dispute Filed, Payment Processed, Carrier Verified
• Each event: icon, description, actor, timestamp (relative)
• Auto-refreshes every 30 seconds
• Filter by event type
• Load more button for older events
• Demo page at frontend/sandbox/admin/page.tsx renders the feed