Add AUTH_TYPE enum and global auth_type variable to auth.h, replacing the FREE_VERSION preprocessor conditional in chat.h with a dynamic get_prompt_model() function that returns "gpt-3.5-turbo" for free users and "gpt-4o-mini" for API key holders. Update all references to prompt_model to use the new function, enabling runtime model switching without recompilation.
Add initial project structure including .gitignore for build artifacts,
Makefile with gcc build and run targets, README placeholder, cgi-bin
directory with gpt.py and gpt_template.html, and core C headers for
chat, http, line input, markdown rendering, and messages.
