Instead of calculating change client-side (which doesn't account for
balance applied, fees, etc.), the app now uses the Change value returned
by the /tasks/complete.php endpoint after processing.
Changes:
- APIService.completeTask now returns CashCompletionResult with backend values
- Added CashCompletionResult struct (cashReceived, orderTotal, change, fees, routing)
- CashCollectionSheet shows confirmed backend change after completion
- Added ratingRequired error case to APIError enum
- Client-side estimate still shown as preview before confirmation
