setMqttProxyActive

Phone-local control over the MQTT proxy that does not touch the device's persisted MQTT config. Turning it off calls MqttManager.stop immediately, cutting the proxy firehose that can saturate the BLE link and MCU on nRF devices — no slow device read-modify-write-readback round-trip. Turning it back on resumes proxying using the device's current MQTT config (a no-op unless the device has MQTT and proxy-to-client enabled).