Gal-Lahat/Function-Aprox — reverse-engineered prompt

Build me a simple C# web app for function approximation. I want to open it in the browser, enter a set of x and y points, or type a basic function over a range, then have the app create an approximate function from that data and show the result visually.

Keep it friendly for a student or someone learning numerical methods. The page should make it clear what values to enter, let me run the approximation, and then show the original points or curve next to the approximated curve. It should also show a small summary with the formula or coefficients if possible, plus a simple error value so I can tell how good the approximation is.

Since there isn’t a useful README, keep the scope small and make reasonable choices based on the existing C# web project. Clean up anything broken, make it easy to run from the solution, and add short instructions for how to start it locally.