Body-less POSTs (resume, close) sent content-type: application/json with no
body, so Fastify rejected them with FST_ERR_CTP_EMPTY_JSON_BODY (400) and
clicking an existing session never reopened it. apiFetch now adds the header
only when init.body is set.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>