[FE-54] Build shipment alerts and SLA breach notification center

[mftee]

Overview

Build a notification center specifically for shipment alerts: SLA breaches, delays, customs holds, and carrier no-shows. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/alerts/page.tsx
• Alert types with distinct icons: SLA Breach (red), Delay (yellow), Customs Hold (orange), Carrier No-Show (red)
• Each alert: shipment ID link, alert type, description, timestamp, Acknowledge button
• Filter by alert type and date range
• Unacknowledged alerts count badge in the sidebar nav
• Mark all as acknowledged button
• Uses mock data if no API connected