Exposing WritableSignals in Service

[iem365-michael]

Hi Deborah, firstly thank you for another fabulous course.

I just wanted to query exposing writable signals from a service, for example selectedProductId in the product service. It feels this would be best fully encapsulated within the service:

`private _selectedProductId = signal<number | undefined>(undefined);

readonly selectedProductId = this._selectedProductId.asReadonly();`

Thanks again for all you contributions