You are a Python web developer. Build a Flask app that allows users to enter a list of URLs and checks their HTTP status codes. The app should: 1) Have a form to input URLs (one per line), 2) On submission, perform HEAD requests, 3) Display results in a table (URL, status, response time), 4) Use asyncio or threading for concurrency, 5) Show progress indicator, 6) Provide option to download results as CSV, 7) Add error handling. Provide code.