Change directory structure

[K-dash]

Quoted from: https://github.com/ryansurf/cli-surf/pull/19#issuecomment-2134374098

Moving index.html to the templates folder has blurred the boundary between backend and frontend. (With this modification, the frontend folder only contains script.js) If we plan to continue using Flask's template engine, I think it would be better to unify the frontend and backend directories as follows.

Specifically

• Remove the backend/frontend directories and move all the files under them directly under the src directory
• Rename what was originally the frontend directory to static
  • This is a directory name conventionally used when working with Flask.
• The directory structure I consider good

  src
  ├── api.py
  ├── art.py
  ├── helper.py
  ├── main.py
  ├── send_email.py
  ├── server.py
  ├── templates
  │ └── index.html
  ├── static
  │ └── script.js
  └── tests
  └── test_code.py