What is the right way to show future startup idea and progress in the resume?

At its current state, your startup idea is still just a side-project. Your prototype only becomes a startup when you have customers/users. You can tag it as work experience but it doesn't really impress anyone if you can't talk about what you're doing in depth.

A resume should be focused on the impact that has materialized in the past, not on a hypothetical future.

However, one way to showcase the potential of what you're working on is by sharing how you've captured demand:

• Waitlist of 2000 users, growing 10% month/month
• Hundreds of pre-launch orders already
• Landing page receives X amount of traffic

You don't need to have a registered company for it to be considered experience. IMO the main difference between a side project and experience is:

• Are you working with a team (work experience) or solo (side project)
• Are you working on it full-time or have the intention to (work experience)