feat: say when the firmware is flashed over USB

Builds with one app slot (C6, debug, and the C5 to come) have nowhere
for /updateFirmware to write. It answered "Error updating firmware: "
and rebooted.

- ConfigSettings::otaSlot, from esp_ota_get_next_update_partition().
- The header sends otaSlot, default 1, so only those builds send it.
- /updateFirmware refuses with 400 and a reason, and doesn't stop the
  radio or reboot.
- The web UI's Update Firmware says to flash the onboard image over USB,
  with a backup first. Firmware before the field leaves it undefined,
  which keeps the uploader.

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

src/ConfigSettings.cpp

@@ -7,6 +7,7 @@
 #include "Utils.h"
 #include "Secrets.h"
 #include "esp_chip_info.h"
+#include "esp_ota_ops.h"
 #include "esp_log.h"
 
 static const char *TAG = "Config";
@@ -144,6 +145,7 @@ bool ConfigSettings::begin() {
       break;
   }
   ESP_LOGD(TAG, "Chip Model ESP32-%s", this->chipModel);
+  this->otaSlot = esp_ota_get_next_update_partition(nullptr) != nullptr;
   this->fwVersion.parse(FW_VERSION);
   uint64_t mac = ESP.getEfuseMac();
   for(int i=0; i<17; i=i+8) {