Need advice on creating a desktop application ready for the web

I need to build a small windows 7 desktop application for workers hour reporting. The application will export the information inputted by the user to an Excel file.

Despite this being a desktop application, there's a possibility that sometime in the future it will become an web application available in the intranet. So i would like to easily do it with little code transformation. Also, the application needs to be installed or unzipped in the local machine with no user configuration except for the installation folder.

I would prefer to use:

• HTML, JS, CSS for the front-end;
• C# (or Java, but i need an Excel library for the export function);

The alternatives i am seeing here are:

1. If build an ASP.net application, but it would require that i include some lightweight web server in my application solution.
2. If i build a Windows Forms application, i would still need to build the web app and somehow embed it.
3. Use some existing platform. Would Xamarin help me here? Or is there any alternative?

I appreciate all your input on which approach would be best, and how to go about it.