Fix/inconsistent hyperlinks 22

[vivekdevaa124]

This PR fixes issue #22 where model hyperlinks were inconsistently displayed as *** or led to the generic vcell.org homepage instead of specific model pages.

Changes:

• System Prompt Update: Added a new 'Biomodel Link Guidelines' section to backend/app/utils/system_prompt.py. This instructs the LLM to always use the bmKey field to construct direct links in the format https://vcell.org/biomodel/<bmKey>.
• Backend Bug Fix: Fixed a deprecated langchain import in backend/app/services/knowledge_base_service.py that was preventing the backend from starting correctly (ModuleNotFoundError: No module named 'langchain.text_splitter').
• Frontend Connectivity: Added NEXT_PUBLIC_API_URL configuration to ensure the chat interface can reach the local backend.

Verification:

• Backend and Frontend servers start cleanly.
• Verified via browser testing that AI responses now include correctly formatted, clickable links (e.g., https://vcell.org/biomodel/271989751) instead of the generic homepage.