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/esp_now_messages.proto

@@ -20,6 +20,8 @@ enum EspNowMessageType {
   ESPNOW_ACCEL_SAMPLE = 12;
   ESPNOW_SET_ACCEL_STREAM = 13;
   ESPNOW_LED_RING = 14;
+  ESPNOW_BATTERY_QUERY = 15;
+  ESPNOW_BATTERY_REPORT = 16;
 }
 
 message EspNowUnicastTest {
@@ -69,6 +71,20 @@ message EspNowAccelSample {
   sint32 z = 4;
 }
 
+/** Master → slave: on-demand LiPo read (optional; slaves also push every ~30 s). */
+message EspNowBatteryQuery {
+  uint32 client_id = 1;
+}
+
+/** Slave → master: LiPo voltages (periodic ~30 s and on query). */
+message EspNowBatteryReport {
+  uint32 client_id = 1;
+  bool lipo1_valid = 2;
+  bool lipo2_valid = 3;
+  uint32 lipo1_mv = 4;
+  uint32 lipo2_mv = 5;
+}
+
 /** Master → slave: LED ring command (same modes as UART LedRingProgressRequest). */
 message EspNowLedRing {
   uint32 client_id = 1;
@@ -121,5 +137,7 @@ message EspNowMessage {
     EspNowAccelSample accel_sample = 13;
     EspNowAccelStream accel_stream = 14;
     EspNowLedRing led_ring = 15;
+    EspNowBatteryQuery battery_query = 16;
+    EspNowBatteryReport battery_report = 17;
   }
 }