Add encryption key management feature with key rotation

[Nitanshu715]

✨ Overview
This PR introduces a secure and fully functional encryption key management system to the Password Manager.
It allows users to change or rotate their master encryption key while keeping all their stored passwords safe — automatically re-encrypting them with the new key behind the scenes.

🧠 Why This Matters
Security-focused applications like a password manager must allow users to:
Periodically rotate encryption keys for better protection.
Recover from compromised keys without losing data.
Have a transparent and automated process for key updates.
This feature directly strengthens the project’s real-world security and user control, aligning perfectly with best practices for encrypted data storage.

⚙️ Technical Details
Added a new module: key_manager.py
Handles key generation, loading, and rotation.
Securely stores the current key in a local master.key file.
Provides a method change_key(old_key, new_key, data_list) to re-encrypt all stored password entries.
Integrated with the existing encryption flow to ensure seamless backward compatibility.
Added error handling for missing or invalid keys.
Updated README.md with clear usage instructions and security notes.

🧩 Example Usage
from key_manager import KeyManager
manager = KeyManager()
new_key = manager.generate_key()
updated_data = manager.change_key(manager.key, new_key, data_list)
save_passwords(updated_data)

🧾 Summary of Changes
✅ Implemented full encryption key lifecycle (generate, load, rotate)
✅ Automatic re-encryption of stored data on key change
✅ Safe file-based key persistence (master.key)
✅ Backward compatible with existing encryption code
✅ Updated documentation and inline code comments

🛡️ Benefits
🔒 Stronger user data protection
🔁 Enables secure key rotation and recovery
⚡ Improves code modularity with clear separation of encryption logic
🧰 Lays foundation for future features (e.g., user-defined passphrase, cloud backup encryption)

🙌 Closing Thoughts

This feature makes the Password Manager more secure, maintainable, and production-ready.
It’s a clean, non-breaking addition that enhances the app’s reliability and long-term usability.