Add UART OTA upload with A/B partition support.

Firmware buffers 200-byte chunks into 4 KiB blocks for esp_ota_write; goTool
uploads with per-block ACK flow control and larger UART buffers to avoid stalls.

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

main/proto/uart_messages.proto

@@ -1,5 +1,7 @@
 syntax = "proto3";
 
+import "nanopb.proto";
+
 package alox;
 
 enum MessageType {
@@ -46,6 +48,8 @@ message EchoPayload {
 message VersionResponse {
   uint32 version = 1;
   string git_hash = 2;
+  /** Active OTA app partition label, e.g. "ota_0" or "ota_1". */
+  string running_partition = 3;
 }
 
 message ClientInfo {
@@ -100,21 +104,25 @@ message EspNowUnicastTestResponse {
   uint32 seq = 2;
 }
 
+// Host → device: begin UART OTA (erase inactive OTA slot; device replies OTA_STATUS).
 message OtaStartPayload {
   uint32 total_size = 1;
-  uint32 block_size = 2;
 }
 
+// Host → device: firmware chunk (up to 200 bytes); device buffers 4 KiB before flash write.
 message OtaPayload {
-  uint32 block_id = 1;
-  uint32 chunk_id = 2;
-  bytes data = 3;
+  uint32 seq = 1;
+  bytes data = 2 [(nanopb).max_size = 200];
 }
 
-message OtaEndPayload {
-  uint32 status = 1; 
-}
+// Host → device: no more payload; device flushes buffer and finalizes OTA.
+message OtaEndPayload {}
 
-message OtaStatusPayload { 
-  uint32 status = 1; 
+// Device → host status (also used as ACK after each 4 KiB written).
+// status: 1=preparing, 2=ready, 3=block_ack, 4=success, 5=failed
+message OtaStatusPayload {
+  uint32 status = 1;
+  uint32 bytes_written = 2;
+  uint32 target_slot = 3;
+  uint32 error = 4;
 }