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/nixie595.ino

@@ -128,7 +128,7 @@ void IRAM_ATTR writeDisplay() {   //timer1 ISR, single-shot: must re-arm timer1
   }
   if (timer < 500) timer = 500;  //a too short timer1 reload can miss the interrupt
 
-  if ((brightness == 0) || (state == 3) || (!radarON)) {
+  if ((brightness == 0) || (state == 3) || (!radarON) || (!tubesOn)) {
     state = 0;
     w = 0;
   }