givingwu/flow-design-center ? reverse-engineered prompt

Build me a simple approval flow chart drawing app where users can visually create and edit a workflow.

It should come with the common nodes already built in, like a start person, approver, parallel branch, condition branch, and cc recipient. I also want to be able to add custom nodes later, so please make the system flexible enough to extend. The editor should let me drag, connect, and rearrange nodes on a canvas in an easy way.

Please include support for two internal data shapes, one like a linked list and one like a flow graph, and make it easy to convert the diagram data into and out of JSON or my own model format. If you need to look up current docs online for anything, go ahead.

Keep the UI clean and simple, and make sure the whole thing feels usable for real approval workflows, not just a demo.