Do you persist any data? Where does business logic reside? How do you validate inputs?

with a backend, data persistence is simply CRUD. for the backendless app, it's all in localstorage, and it can be downloaded into a file as backup. all the logic is in the browser, just like any desktop application. you should not need a server to manage personal data that is stored on your computer.

of course it is a tradeoff. if the user wanted to save their data outside the browser every few minutes then that would not be practical or if there is to much new data being generated so it doesn't fit into localstorage. i'd either need a server (even if running locally) that can save that data without user intervention or i should not use the browser as a development platform. in such cases a browser based SPA would have been the wrong choice. maybe electron, or a traditional desktop framework