Design and Develop a Web App and a Desktop App for a VPN

PLEASE NOTE: We would like to discuss the budget with you directly if you are interested. Please message me if you have any questions. We are a VPN service currently running in a very "alpha" state. We would like to upgrade our front-end infrastructure mainly consisting of our custom OpenVPN GUI and billing/invoicing web app. The full description of our current infrastructure and our plans is available here: [login to view URL] In short: We need to upgrade our GUI to use OpenVPN management interface (telnet like interface used to manage OpenVPN service) instead of just launching and killing a binary which is less stable. We would also like the app to run on OS X as well as Windows. Preferred solution - C++/QT , C# or node.js / electron app. We need to upgrade our web client area which is currently WHMCS which is not very suitable for a VPN (At least as a front-end). We want it to be much simpler. This will likely force us to rebuild (at least partially) our user authentication infrastructure as we are currently using RADIUS which we are not very happy with. Preferred solution - PHP or other app with Google App Engine compatibility.