Allow members to participate only in expense management

[ceple97]

Not every participant needs full access to trip planning.

It would be useful if members could join only for expense tracking.

Example permissions:

• ✅ View balances
• ✅ Add expenses
• ✅ Mark payments
• ✅ View settlement
• ❌ Edit itinerary
• ❌ Edit bookings
• ❌ Change trip settings
• ❌ Delete content

This would make TREK a viable replacement for Splitwise during group trips while keeping itinerary management in the hands of the organizers.