Refactor Streamlit app and enhance backend management

- Refactored app.py to modularize the code, significantly reducing its size and improving maintainability.
- Introduced new backend modules for chat management, model management, and LLM interface, enhancing separation of concerns.
- Removed the PDF extraction summary and RAG implementation documentation as part of the cleanup.
- Improved session state initialization for better management of chat and model states.
- Updated the main application logic to utilize the new backend structure, ensuring a smoother user experience.

Author: clstaudt