[Frontend] Build New Shipment Form Page

[mftee]

Description:
Build the multi-section form for shippers to create a new shipment. The form uses react-hook-form + zod and is grouped into logical card sections: Route, Cargo Details, Pricing, Schedule & Notes.

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

Acceptance Criteria:

• 'use client' directive
• Zod schema validates all fields — see CreateShipmentDto for rules
• Form sections (each in a <Card>):
  1. Route — origin, destination (side by side on sm+)
  2. Cargo Details — description textarea (min 10), weight, optional volume
  3. Pricing — price, currency (default USD)
  4. Schedule & Notes — pickupDate, estimatedDeliveryDate (datetime-local inputs), notes textarea
• Submit calls shipmentApi.create(payload), then toast.success + router.push(/shipments/${shipment.id})
• On error: toast.error()
• Submit button disabled + shows "Creating…" while submitting
• Cancel button calls router.back()
• All field errors shown inline below the relevant input