feat(nix,ci): ci for automated test app, use flakes instead of raw mkshell

Hi @codenoid , This pr does the followings, 1. Removing old raw `mkshell` in favor of `flakes` to exactly lock the dependencies. 2. Github ci to automatically test and build the app on push event on `main` branch. AND: U ain't wrong tho 😭 <img width="234" alt="Screenshot 2024-08-08 at 3 57 30 PM" src="https://github.com/user-attachments/assets/71178624-066b-45e5-8d99-17d2c12c2e37"> Let me know if you'd like to add/remove anything I'd be happy to follow u up.