UX: validate that there is indeed a file loaded before build

<img width="972" alt="image" src="https://github.com/String-sg/chergpt-starter-kit/assets/44336310/674713a2-b18d-4514-861b-61706211e0ca">

issue: user can press create and wait indefinitely without adding a file to RHS

fix: to validate/ throw an error if no files loaded 