Fix web OTA upload and isolate OTA sessions across firmware and goTool.

Split ESP-NOW into core/master/slave modules, block non-OTA UART traffic during updates, and hold the host serial port exclusively so dashboard polling cannot interleave with firmware uploads.

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

goTool/api_serve.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -146,7 +147,11 @@ func serveOTAUpload(w http.ResponseWriter, r *http.Request, link *managedSerial,
 		if hub != nil {
 			hub.broadcastRaw(OTAProgress{Type: "ota_progress", Phase: "error", Message: err.Error()})
 		}
-		writeJSON(w, http.StatusServiceUnavailable, otaAPIResponse{Error: err.Error()})
+		status := http.StatusServiceUnavailable
+		if errors.Is(err, errOTAInProgress) {
+			status = http.StatusConflict
+		}
+		writeJSON(w, status, otaAPIResponse{Error: err.Error()})
 		return
 	}
 	writeJSON(w, http.StatusOK, otaAPIResponse{