[Frontend] Build Shipments List Page with Status Filter Tabs

[mftee]

Description:
Build the main shipments list page that shows the authenticated user's shipments (role-filtered by the backend). Include tabbed filtering by status and a "New Shipment" button for shippers.

Create frontend/app/(dashboard)/shipments/page.tsx.

Acceptance Criteria:

• 'use client' directive
• Fetches shipments via shipmentApi.list({ status }) on mount and on tab change
• Status tabs: All, Pending, Accepted, In Transit, Delivered, Completed (horizontal tab row with bottom border indicator)
• Shows loading skeleton (4 animated pulse cards) while fetching
• Shows empty state when no results — with "Create your first shipment" CTA for shippers if "All" tab is selected
• Renders a <ShipmentCard> per shipment in a responsive grid (1 col mobile, 2 col sm+)
• Shows total count below the grid: "Showing X of Y shipments"
• "New Shipment" button (top right) visible only to shippers / admins
• Uses toast.error() on fetch failure