[Frontend] Update Dashboard Layout with Role-Based Navigation

[mftee]

Description:
Replace the "Navigation coming soon" placeholder in the dashboard layout with a real, functional sidebar navigation that shows different links depending on the user's role.

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

Acceptance Criteria:

• 'use client' directive (needs usePathname and auth store)
• Navigation items by role:
  • Shipper / Admin: Dashboard, My Shipments, Create Shipment
  • Carrier: Dashboard, My Jobs, Marketplace
• Active link detection: exact match for /dashboard; startsWith for all others
• Active link styled with bg-primary/10 text-primary; inactive with text-muted-foreground hover:bg-accent hover:text-foreground
• User footer at the bottom of the sidebar: avatar initials (first letter of first + last name), full name, role (capitalized), "Sign out" button
• Sign out calls useAuthStore().logout()
• No breaking changes to the overall flex min-h-screen layout structure