Websites are apps, apps are websites. The area between each is a gradient, not black and white. The best way to measure this is to check a couple of metrics.

Does this resource need to be generated dynamically?

Related: can this resource be cached?

More important: can this resource be resized?

Often overlooked: do I need to process this javascript to provide the equivalent UX?

The modern Web is terrible, and the modern state of app dev is terrible due to a substantial subset of the reasons that the modern Web is terrible.