Detect search query to start webchat

[Mardak]

Like https://github.com/ggerganov/llama.cpp/pull/6554 to support http://localhost:8080/?q=prompt like https://chat.openai.com/?q=prompt starting a chat but without the index.html.hpp nor fix to api_url not applicable here.

(cherry picked from commit ggerganov/llama.cpp@400d5d722d7edf7de0cf24a18c42b183c65047d2)