/api/queue now returns a "device" object: last_seen/overdue, running and
available firmware versions, battery percent + on-battery duration +
linear-fit remaining-time estimate (recharge cycles reset the history so
estimates never span a charge). New POST /api/firmware (token-gated
upload, validates the embedded esp_app_desc_t) and GET /frame/firmware
(token-gated download) let a build be pushed to the device without
touching it physically. GET /frame/config now accepts an X-Frame-Version
header and returns the available firmware version, piggybacking the
device's update check on a request it already makes every wake.