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/ProtoSchema.h

@@ -169,6 +169,7 @@
   FIELD(hostname, "hn", 's', 0) \
   FIELD(model, "md", 's', 0) \
   FIELD(chipModel, "cp", 's', 0) \
+  FIELD(otaSlot, "os", 'b', 1) \
   FIELD(connType, "ct", 's', 0) \
   /* memory, wifi and ethernet */ \
   FIELD(freeHeap, "fr", 'i', 0) \