refactor(mqtt): migrate observer settings to MQTTPrefs structure

Refactored the handling of observer-related settings by moving them from
NodePrefs to a new MQTTPrefs structure. This change centralizes MQTT,
WiFi, timezone, SNMP, and alert configurations, improving code organization
and maintainability. The new structure allows for better separation of
concerns and prepares the codebase for future enhancements.

src/Dispatcher.cpp

@@ -39,9 +39,11 @@ uint32_t Dispatcher::getCADFailMaxDuration() const {
   return 4000;   // 4 seconds
 }
 
+#ifdef WITH_MQTT_BRIDGE
 uint32_t Dispatcher::getRadioWatchdogMillis() const {
   return RADIO_WATCHDOG_MS;
 }
+#endif
 
 void Dispatcher::loop() {
   if (millisHasNowPassed(next_floor_calib_time)) {
@@ -63,10 +65,9 @@ void Dispatcher::loop() {
   }
 
   // Radio watchdog: detect radio stuck in RX mode but not receiving any packets.
-  // Use a composite "last radio activity" timestamp: the most recent of any valid RX,
-  // any ISR event (even CRC errors), or any successful TX.  This prevents false firings
-  // in quiet mesh environments where packets may be more than RADIO_WATCHDOG_MS apart,
-  // while still catching a truly stuck radio (PSRAM starvation → missed ISR → no activity).
+  // Observer-only feature (gated behind WITH_MQTT_BRIDGE); configured via the
+  // MQTTPrefs radio_watchdog_minutes setting.
+#ifdef WITH_MQTT_BRIDGE
   {
     const uint32_t watchdog_ms = getRadioWatchdogMillis();
     if (watchdog_ms > 0) {
@@ -87,6 +88,7 @@ void Dispatcher::loop() {
       }
     }
   }
+#endif // WITH_MQTT_BRIDGE (radio watchdog)
 
   if (outbound) {  // waiting for outbound send to be completed
     if (_radio->isSendComplete()) {