Checks whether llama-server is already healthy on startup and spawns it
from configured binary/model paths if not, polling until ready. Server
infra config (binary, model path, host, port, context size) split out
of prompts.toml into its own server.toml, and all of it lives in a new
server module rather than inline in main.rs, alongside a single reused
HTTP client and a shared health-check helper. Gemma client setup now
runs concurrently with the server health-check/spawn since they're
independent.
