Refactors the application to support multiple users for timetable logging.
- User accounts added via a Discord command are now stored in `accounts.json`.
- The main scheduling loop now fetches timetables for all registered users, in addition to the default user defined in the `.env` file.
- Notifications for registered users are now sent as direct messages, providing personalized updates. The default user continues to use the existing webhook.
- User-specific data files (e.g., `timetable_filled_<userID>.json`) are now used to keep each user's data separate.
- Error handling has been improved to prevent the application from crashing if an operation fails for a single user.
