Add LiPo battery monitoring with ESP-NOW cache and dashboard API.

Slaves report pack voltages every 30s; the master caches them for fast
BATTERY_STATUS reads. goTool exposes REST/WebSocket and shows values in
the dashboard, with a nanopb fix so optional lipo submessages encode.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/proto/uart_messages.proto

@@ -24,6 +24,7 @@ enum MessageType {
   RESTART = 23;
   ACCEL_SNAPSHOT = 24;
   ACCEL_STREAM = 25;
+  BATTERY_STATUS = 26;
 }
 
 message UartMessage {
@@ -54,6 +55,8 @@ message UartMessage {
     AccelSnapshotResponse accel_snapshot_response = 24;
     AccelStreamRequest accel_stream_request = 25;
     AccelStreamResponse accel_stream_response = 26;
+    BatteryStatusRequest battery_status_request = 27;
+    BatteryStatusResponse battery_status_response = 28;
   }
 }
 
@@ -130,6 +133,33 @@ message AccelStreamResponse {
   uint32 slaves_updated = 4;
 }
 
+/** Host → master: read LiPo ADC voltages (master local and/or slaves via ESP-NOW). */
+message BatteryStatusRequest {
+  /** 0 = master only; >0 = one slave; ignored when all_clients */
+  uint32 client_id = 1;
+  /** Master (client_id 0) plus every registered slave */
+  bool all_clients = 2;
+}
+
+message LipoReading {
+  bool valid = 1;
+  /** Estimated pack voltage in millivolts from ADC */
+  uint32 voltage_mv = 2;
+}
+
+message BatterySample {
+  uint32 client_id = 1;
+  LipoReading lipo1 = 2;
+  LipoReading lipo2 = 3;
+  /** Milliseconds since last ESP-NOW battery report from this pod. */
+  uint32 age_ms = 4;
+}
+
+message BatteryStatusResponse {
+  bool success = 1;
+  repeated BatterySample samples = 2 [(nanopb).max_count = 17];
+}
+
 // Host → master: read cached accel samples from slaves (only while stream enabled).
 // client_id 0 = all registered slaves; otherwise one slave.
 message AccelSnapshotRequest {