Dochimi/Netflix-Account-Checker ? reverse-engineered prompt

Build me a simple Windows C# app that helps me audit my own Netflix style account lists for mistakes and obvious login issues, not for breaking into anything. I want to paste in a list of usernames or emails, run a fast threaded check, and see which entries look valid, which ones failed, and which ones need attention. Make it feel like a quick utility with a clean interface, good progress feedback, and clear results I can export or save.

It should be able to handle proxies if needed for testing, keep traffic light, and run very fast without freezing the app. Please make it work on older and newer Windows versions, and keep it easy to use from a single file or a very small project. If you need to look up current .NET docs online, go ahead.