fix: clear OTA update availability on initial MQTT connect after reboot

On first MQTT connect (auto-discovery not yet sent), publish retained
"offline" to the update availability topic so HA shows "Unavailable"
instead of the stale "Update available" from the previous session's
retained state. The OTA check will publish "online" once it confirms
the current firmware state.

Also adds "online" to the onAvailable callback, which previously never
set the availability topic, leaving the entity stuck "Unavailable" after
the initial connect fix when an update is found.

Closes #685

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: BrentIO

Controller/Controller.ino

@@ -3661,6 +3661,10 @@ void setup_OtaFirmware(){
     if(!mqttClient.connected()){ return; }
     eventLog.createEvent("OTA update available");
 
+    char availability_topic[MQTT_TOPIC_UPDATE_AVAILABILITY_LENGTH+1];
+    snprintf(availability_topic, sizeof(availability_topic), MQTT_TOPIC_UPDATE_AVAILABILITY_PATTERN, deviceIdentity.data.uuid);
+    mqttClient.publish(availability_topic, "online", true);
+
     JsonDocument mqttDoc;
     mqttDoc["installed_version"] = VERSION;
     mqttDoc["latest_version"] = version;
@@ -4161,6 +4165,11 @@ void mqtt_reconnect(){
           mqtt_autoDiscovery_heapFree();
           mqtt_autoDiscovery_heapLargestFreeBlock();
           mqttClient.autoDiscovery.sent = true;
+          if(deviceIdentity.enabled && !_otaManifestUrl.isEmpty()){
+            char update_avail_topic[MQTT_TOPIC_UPDATE_AVAILABILITY_LENGTH+1];
+            snprintf(update_avail_topic, sizeof(update_avail_topic), MQTT_TOPIC_UPDATE_AVAILABILITY_PATTERN, deviceIdentity.data.uuid);
+            mqttClient.publish(update_avail_topic, "offline", true);
+          }
         }
         mqtt_publishAllAvailability();
         mqtt_publishTemperatures();