Simplifies architecture by using the existing trips#update route for
sharing settings management instead of a separate route.
## Changes
**Routes**
- Removed: PATCH /trips/:id/sharing → shared/trips#update
- Now uses: PATCH /trips/:id (existing route) with sharing params
**Controllers**
- Shared::TripsController: Simplified to only handle public view (show)
- TripsController: Added update_sharing private method to handle
sharing params when present
**Views**
- Updated JavaScript in _sharing.html.erb to use trip_path with
nested sharing params
**Tests**
- Updated request specs to use trip_path instead of sharing_trip_path
- All params now nested under sharing key
## Benefits
- Cleaner namespace separation (Shared:: only for public access)
- Follows Rails conventions (one update route handles everything)
- Simpler routing structure
- Reduced code duplication
## Backwards Compatibility
This is a breaking change for the sharing API endpoint, but since
this feature was just implemented and hasn't been released yet,
no migration path is needed.
