Add runtime UV protection toggle and treat near-zero sense as external power.

Per-pod NVS enable (default off) via UART/ESP-NOW/dashboard, and skip UV latch when LiPo ADC reads ~0 mV so USB/external supply still boots.

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

main/proto/uart_messages.proto

@@ -47,6 +47,11 @@ enum MessageType {
    * (best_effort / fast / wait / reliable).
    */
   INPUT_SEND_MODE = 35;
+  /**
+   * Host → master: get/set software LiPo UV protection enable (NVS per pod).
+   * client_id 0 = master; >0 = one slave; all_clients = master + every slave.
+   */
+  BATTERY_UV = 36;
 }
 
 message UartMessage {
@@ -92,6 +97,8 @@ message UartMessage {
     ClientIdMapResetResponse client_id_map_reset_response = 43;
     InputSendModeRequest input_send_mode_request = 44;
     InputSendModeResponse input_send_mode_response = 45;
+    BatteryUvRequest battery_uv_request = 46;
+    BatteryUvResponse battery_uv_response = 47;
   }
 }
 
@@ -122,6 +129,8 @@ message ClientInfo {
   bool tap_notify_single = 9;
   bool tap_notify_double = 10;
   bool tap_notify_triple = 11;
+  /** Master cache: software UV protection enabled on this slave (NVS on device). */
+  bool uv_protection_enabled = 12;
 }
 
 message ClientInfoResponse { 
@@ -202,6 +211,23 @@ message InputSendModeResponse {
   uint32 slaves_updated = 4;
 }
 
+// write=false: read UV enable; write=true: persist enable (and clear latch when disabling).
+// client_id 0 = local master; >0 = one slave; all_clients = master + every registered slave.
+message BatteryUvRequest {
+  bool write = 1;
+  bool enabled = 2;
+  uint32 client_id = 3;
+  bool all_clients = 4;
+}
+
+message BatteryUvResponse {
+  uint32 client_id = 1;
+  bool success = 2;
+  bool enabled = 3;
+  bool latched = 4;
+  uint32 slaves_updated = 5;
+}
+
 /** 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 */