Add refresh button to grades display in app.py for improved user experience

Fix variable reference in student ID display within main function of app.py

Refactor database connection parameters to use simplified secret keys for improved clarity and maintainability.

Update database configuration to use simplified secret keys for connection parameters.

Update requirements.txt to include mysql-connector-python and python-dotenv for database connectivity and environment variable management.

Refactor data loading to use MySQL database connection; removed CSV decryption function and implemented student grade retrieval from the database.

modified: app.py

modified: app.py

added grade <= 3.0

updated grade

modified: encrypted_grades.csv

modified: encrypted_grades.csv

modified: app.py

Delete fernet_key.txt