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>

main/esp_now_slave.h

@@ -0,0 +1,14 @@
+#ifndef ESP_NOW_SLAVE_H
+#define ESP_NOW_SLAVE_H
+
+#include "client_registry.h"
+#include "esp_err.h"
+#include "esp_now.h"
+
+esp_err_t esp_now_slave_start(void);
+void esp_now_slave_on_recv(const esp_now_recv_info_t *info, const uint8_t *data,
+                           int len);
+
+bool esp_now_slave_get_master_mac(uint8_t mac_out[CLIENT_MAC_LEN]);
+
+#endif