Add: Simple CustomTkinter login example

[semendeZ0]

Description

Added a new Python desktop application script that provides a modern login interface using CustomTkinter. It features real SQLite database authentication, secure password storage using SHA-256 hashing, and a separate user registration window. 

This serves as a great practical example for beginners learning GUI building, database integration, and basic security/cryptography concepts in Python.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code cleanup/refactoring

Related Issues

N/A

Testing

Please describe the tests you've run to verify your changes:

• Manual testing
• Added automated tests
• Existing tests pass

Details: 
Performed manual testing locally. Verified the successful creation of the database.db file, tested the registration of new users, confirmed that passwords are encrypted properly using SHA-256 in the database, and validated the login authentication (testing both correct and incorrect credentials).

Checklist

• My code follows the project's code style
• I have commented my code, particularly in hard-to-understand areas
• I have updated the README.md if needed
• My changes generate no new warnings
• I have tested my code locally
• New and existing tests pass with my changes

Screenshots (if applicable)