Restructure JSON data hierarchy

[Ymanawat]

The current JSON data structure needs enhancements for better maintainability and future features.

I would like these things to be added :
made username a uniqueID of the user as all platforms
User's Name, Email, password, and more things you can advice. Also for folder and item_type look at the issue https://github.com/Ymanawat/Coursefy/issues/1 , also want to add files in the courses like some codes or algorithms.

Current structure:

{
  "username": "user_name"
  "courses": [
    {
      "key": "unique_course_key",
      "name": "Course Name",
      "description": "Course Description",
      "deadline": "Course Deadline",
      "total_items": 0,
      "completed_items": 0,
      "items": [
                   {
                     "key": "unique_item_key",
                     "title": "Item Title",
                     "url": "Item URL",
                     "completed": false
                   }
         ]
      }
   ]
}