Enhance RAG system integration and user interface in app.py

- Removed unused context checking logic and session state variables to streamline the application.
- Introduced global model and embedding model selectors in the sidebar for improved user experience.
- Updated RAG settings to allow dynamic configuration of chunk size, overlap, and similarity threshold.
- Implemented document loading functionality in the RAG system, enabling users to switch between documents seamlessly.
- Enhanced chat management to handle ongoing generation state when switching or deleting chats.
- Improved error handling and user feedback for document processing and RAG system readiness.

Author: clstaudt