Replaces check_server_reachable() (bare bool, GET /health) with
fetch_frame_config(), which GETs the server's new /frame/config endpoint
instead -- doubles as the reachability check (any completed HTTP response
counts, same as before) and delivers the server-configured
refresh_interval_s, used for the success-path deep sleep duration instead
of the Kconfig-only default.
Parses the tiny JSON response with a hand-rolled scalar extractor
(json_extract_uint) rather than pulling in a JSON library -- cJSON isn't
bundled in this ESP-IDF install, and a single flat integer field doesn't
justify a new dependency. Verified standalone against exactly the JSON
shape the server emits, including a missing-field fallback case.
FRAME_SLEEP_INTERVAL_S (Kconfig) is now just the fallback used before the
device has ever reached a configured server, or if the response is
missing/unparseable -- documented as such in its help text.
tfaour
21af41ff66