Allow user to set the system prompt

Useful for having the LLM take on a role and act as a coach, therapist, programmer, etc. The current system prompt is not great.

Look for where the system prompt is currently defined in the codebase, likely in a configuration or initialization file for the LLM. The change involves adding a user interface element (like a text input or settings panel) to accept a custom prompt and passing it to the model. Check existing UI components for settings to see the pattern. Verify the change by running the app and testing that the new prompt influences the chatbot's responses.

Rédigé par le modèle d'indexation à partir du texte de l'issue.