As the title states, it would probably be an intuitive feature to have a “+” button within categories (maybe on hover it appears) that allows you to quickly add a project to that category. It’s a little clunky to add it from the bottom of the left sidebar IMO – and then to have to find the project as well.
Edit: On right-click is fine for now but not as intuitive either since right-click features are more hidden in comparison.