feat: public read-only share links with permissions — closes #79

Share links:
- Generate a public link in the trip share modal
- Choose what to share: Map & Plan, Bookings, Packing, Budget, Chat
- Permissions enforced server-side
- Delete link to revoke access instantly

Shared trip page (/shared/:token):
- Read-only view with TREK logo, cover image, trip details
- Tabbed navigation with Lucide icons (responsive on mobile)
- Interactive map with auto-fit bounds per day
- Day plan, Bookings, Packing, Budget, Chat views
- Language picker, TREK branding footer

Technical:
- share_tokens DB table with per-field permissions
- Public GET /shared/:token endpoint (no auth)
- Two-column share modal (max-w-5xl)

client/src/components/shared/Modal.tsx

@@ -7,6 +7,7 @@ const sizeClasses: Record<string, string> = {
   lg: 'max-w-lg',
   xl: 'max-w-2xl',
   '2xl': 'max-w-4xl',
+  '3xl': 'max-w-5xl',
 }
 
 interface ModalProps {