Visibility controls for products and specs

Products and specs now support public/private visibility settings. You can mark a product as public or private when creating or editing it, and the product list shows each product’s visibility. A product’s visibility is inherited by its specs unless a spec has its own explicit visibility override, so a public spec can be exposed even when attached to a private product, and a private spec can be hidden even when attached to a public product. A new product settings page also lets you edit the product name, description, visibility, and linked GitHub repository.