Flash the Nano over WiFi through the ESP32

- ESP32: NanoFlashBridge on TCP port 2323. A connection power cycles the
  Nano into its bootloader and bridges TCP <-> Serial2 at 57600 baud; the
  clock pauses during the session, 10 s idle timeout
- tools/nano_net_upload.py: STK500v1 uploader over TCP (standard library
  only), with sync retries, write and read-back verification. avrdude 6.3
  net: ports are broken on Windows, so avrdude is not used
- New nano_ota environment: pio run -e nano_ota -t upload
- README: usage, requirements, troubleshooting; TODO for hardware test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/libs.h

@@ -1,6 +1,7 @@
 #include <Arduino.h>
 #include "BROSE9323.h"
 #include "bitmaps.h"
+#include "nano_flash_bridge.h"
 
 // WiFi / MQTT credentials (gitignored, copy secrets.example.h -> secrets.h)
 #if __has_include("secrets.h")
@@ -52,6 +53,12 @@ const int nano_tx = 17; // UART2 TX, wired to the Nano RX
 bool relay_on = false;
 // Max wait for the Nano's READY after power on (also the wait when Nano TX is not wired to our RX)
 const uint32_t NANO_BOOT_TIMEOUT = 3000;
+const uint32_t NANO_BAUD = 115200;
+
+// Flashing the Nano over WiFi (see README.md, pio run -e nano_ota -t upload)
+const uint16_t NANO_FLASH_PORT = 2323;
+const uint32_t NANO_BOOTLOADER_BAUD = 57600; // old Nano bootloader
+const uint32_t NANO_POWER_OFF_TIME = 1000;   // relay off time to make sure the Nano resets
 
 int hour = 0;
 int minute = 0;