Support both Nano bootloaders for flashing over WiFi

- Uploader sends 'FLASH <baud>' first; the ESP32 restarts the Nano and
  bridges at that baud rate, then answers OK
- --baud=auto (default) tries 57600 (old ATmegaBOOT) then 115200 (Optiboot)
- Optional nano_reset GPIO (NPN to Nano RESET) pulses reset after power
  on, needed for Optiboot which skips the bootloader after a power cycle
- Clear errors for outdated ESP32 firmware and missing wires
- README: bootloader table, reset wire, troubleshooting

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

src/esp32/main.cpp

@@ -60,18 +60,29 @@ void relayOff()
   pinMode(nano_tx, INPUT);
 }
 
-// Flash session started: power cycle the Nano so its bootloader runs, and talk to the
+// Flash session started: restart the Nano so its bootloader runs, and talk to the
 // bootloader at its baud rate. The PC uploader keeps sending sync bytes until it answers.
-void startNanoFlash()
+// The old bootloader also starts after a power cycle; Optiboot only after a reset pulse.
+void startNanoFlash(uint32_t baud)
 {
   if (relay_on)
     relayOff();
   delay(NANO_POWER_OFF_TIME);
 
-  nanoSerial.begin(NANO_BOOTLOADER_BAUD, SERIAL_8N1, nano_rx, nano_tx);
+  nanoSerial.begin(baud, SERIAL_8N1, nano_rx, nano_tx);
+  digitalWrite(relay, LOW);
+
+  if (nano_reset >= 0)
+  {
+    delay(NANO_RESET_DELAY);
+    digitalWrite(nano_reset, HIGH);
+    delay(NANO_RESET_PULSE);
+    digitalWrite(nano_reset, LOW);
+  }
+
+  // drop anything the Nano sent before its bootloader started
   while (nanoSerial.available())
     nanoSerial.read();
-  digitalWrite(relay, LOW);
 }
 
 // Flash session ended: switch off, the clock powers the Nano on again with the new firmware
@@ -312,6 +323,11 @@ void setup()
   otaPanicMode = false;
 
   pinMode(relay, OUTPUT);
+  if (nano_reset >= 0)
+  {
+    digitalWrite(nano_reset, LOW);
+    pinMode(nano_reset, OUTPUT);
+  }
   relayOn();
 
   display.begin(&nanoSerial);