feat(power): add battery monitoring and status overlays

components/control_plane/include/control_plane.h

@@ -88,6 +88,15 @@ typedef struct {
 
 esp_err_t control_plane_init(void);
 
+/**
+ * @brief Start battery sensing and charge-status presentation.
+ *
+ * Safe to call repeatedly. This service is intentionally started before
+ * connectivity bootstrap so battery and charger indications do not depend on
+ * Wi-Fi, provisioning, or server readiness.
+ */
+esp_err_t control_plane_battery_service_start(void);
+
 /**
  * @brief Prepare platform-backed persistent storage before runtime workers start.
  *