[Frontend] Update Dashboard Page with Real Shipment Metrics

[mftee]

Description:
Update the dashboard home page to replace the "—" placeholder metric cards with real data fetched from the shipments API. Add a "Recent Shipments" section below the metrics.

Update frontend/app/(dashboard)/dashboard/page.tsx.

Acceptance Criteria:

• Fetches user data + shipment stats on mount (parallel requests):
  • shipmentApi.list({ status: 'in_transit', limit: 1 }) → active count
  • shipmentApi.list({ status: 'pending', limit: 1 }) → pending count
  • shipmentApi.list({ status: 'completed', limit: 1 }) → completed count
  • shipmentApi.list({ limit: 4 }) → recent 4 shipments
• Metric cards show real numbers from the API (fallback "—" while loading)
• Metric card labels adapt to role: Carriers see "Active Jobs" / "Awaiting Pickup"; Shippers see "Active Shipments" / "Awaiting Carrier"
• "Recent Shipments" section with "View all →" link; renders <ShipmentCard> for each
• Empty state in recent section: role-appropriate CTA ("Create your first shipment" for shippers, "Open Marketplace" for carriers)
• Header CTA button: Shippers see "+ New Shipment" (links to /shipments/new); Carriers see "Browse Marketplace" (links to /marketplace)
• Sign out button removed from header (now in sidebar)