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>

data-src/js/model.js

@@ -140,7 +140,7 @@ ProtoModel.ITEMS = [
 // have it.
 ProtoModel.FIELDS = {
     header: ['maxRooms', 'maxShades', 'maxGroups', 'maxGroupedShades', 'maxLinkedRemotes', 'startingAddress', 'bitLength',
-        'proto', 'serverId', 'hostname', 'model', 'chipModel', 'fw', 'connType'],
+        'proto', 'serverId', 'hostname', 'model', 'chipModel', 'otaSlot', 'fw', 'connType'],
     room: ['roomId', 'name', 'sortOrder'],
     shadeState: ['shadeId', 'position', 'target', 'direction', 'myPos', 'tiltPosition', 'tiltTarget', 'tiltDirection',
         'myTiltPos', 'flags', 'lastRollingCode'],