Refactor: Apply Rails best practice for early returns

Follow Rails convention of using "render/redirect ... and return"
instead of standalone return statements in controller actions.

## Changes

**Shared::TripsController#show**
Before:
```ruby
unless @trip&.public_accessible?
  return redirect_to root_path, alert: '...'
end
```

After:
```ruby
redirect_to root_path, alert: '...' and return unless @trip&.public_accessible?
```

**TripsController#update**
Before:
```ruby
if params[:sharing]
  return update_sharing
end
```

After:
```ruby
update_sharing and return if params[:sharing]
```

## Benefits
- More idiomatic Rails code
- Clearer intent with single-line guard clauses
- Prevents potential double render issues
- Follows community best practices

app/controllers/shared/trips_controller.rb

@@ -4,10 +4,7 @@ class Shared::TripsController < ApplicationController
   def show
     @trip = Trip.find_by(sharing_uuid: params[:trip_uuid])
 
-    unless @trip&.public_accessible?
-      return redirect_to root_path,
-                         alert: 'Shared trip not found or no longer available'
-    end
+    redirect_to root_path, alert: 'Shared trip not found or no longer available' and return unless @trip&.public_accessible?
 
     @user = @trip.user
     @is_public_view = true

app/controllers/trips_controller.rb

@@ -40,9 +40,7 @@ class TripsController < ApplicationController
 
   def update
     # Handle sharing settings update (JSON response)
-    if params[:sharing]
-      return update_sharing
-    end
+    update_sharing and return if params[:sharing]
 
     # Handle regular trip update
     if @trip.update(trip_params)