Structure for a spotify / OAUTH web app

Question

I am creating a spotify app that will allow 2 users to compare what songs they have in common. I have never designed a super-complicated web app before, so I wanted to get some advice for the structure of how this works. I created a flowchart describing how I felt the site should work.

Taking into security and possible error points, what changes can I make to my flowchart / website design to develop the most robust app possible (even if it is a simple one)?

I have never posted on softwareengineering before, so if this is the wrong forum or more information is needed, please let me know instead of downvoting.