Add Home Assistant controls over MQTT

Upgrade ArduinoHA from 1.3.0 to 2.1.0 and rewrite mqtt.ino for it.
The clock now shows up in Home Assistant (discovery prefix
"homeassistant") with:
- switches Tubes (blank the display, runtime only) and Auto dim
- numbers Day/Night brightness
- light Backlight (on/off, brightness, RGB mapped to the colour wheel)
- select LED effect, button Cathode protect
- sensors Light (LDR) and WiFi signal, plus the upstream sensors when
  the clock has them; master-clock subscriptions are kept

Settings changed from HA are saved to EEPROM 3 s after the last change.
Fixes the unbounded memcpy in the MQTT message handler.

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

firmware/uniclock/ESP32_UniClock2/clocks.h

@@ -141,6 +141,8 @@ byte tubePixels[] = {0, 0, 1, 1, 2, 2, 3, 3};   //8 WS2812, two per tube (strip
 #define AP_NAME "UNICLOCK"
 #define AP_PASSWORD ""
 #define WEBNAME "IN-12 Nixie Clock"
+#define USE_MQTT              //Home Assistant: broker address/user/password on the web page, then reboot
+#define MQTT_PREFIX "homeassistant"   //Home Assistant's default discovery prefix
 #define FIRMWARE_SERVER ""    //no web-triggered HTTP update; use ArduinoOTA
 #define USE_ARDUINO_OTA
 #define OTA_HOSTNAME "nixie-in12"