[demo] Add product details screen

[breedx-splk]

When the astronomy shop user is looking at the list of products, they should be able to click one and view the details. The details page should show a larger picture of the item, its name, full description, and price.

The user should be able to specify a quantity (via spinner?) of items to add to their cart. The cart doesn't yet exist, so it can either be created as part of this effort (very basic) or can be stubbed.

There should also be a button that allows the user to return to the main product list. In the future, there will be related products as well, but that can be skipped for now.

[breedx-splk]

This is implemented! Thanks @magda-woj !