Build a simple UI

[HenryQuan]

Add a simple UI so it is easier to use by prodiving options to pick different engine and backend.

• Support the console mode
• Support single file mode
• Support project migration mode

More backends should be added or provide a generic one for curly bracket languages. Maybe use Flet?

[HenryQuan]

The implementation is quite straight forward, if any Flutter packages supports what I am using (different base url & openai), I can simply convert the Python project to Flutter directly.