ha_discovery: publish state/availability under home_automation prefix

Follow the standard MQTT Discovery split: discovery config stays under the HA
discovery prefix (homeassistant/.../config, required), but the actual state and
availability are published under our own prefix (default home_automation/). The
config payload's state_topic/availability fields point at the new prefix so HA
reads state from there.

- new config ha_state_topic_prefix (default home_automation).
- build_discovery_payload + all publish_* split discovery_prefix (config topic)
  from state_prefix (state/availability). energy-cost still omits availability;
  modbus availability structure unchanged. Tests updated.

app/services/config_page.py

@@ -122,6 +122,12 @@ CONFIG_FIELDS: tuple[ConfigField, ...] = (
         "ha_discovery_prefix",
         "HA Discovery Prefix",
     ),
+    ConfigField(
+        "Home Assistant Discovery",
+        "HA_STATE_TOPIC_PREFIX",
+        "ha_state_topic_prefix",
+        "HA State Topic Prefix",
+    ),
     ConfigField("Modbus", "MODBUS_POLLING_ENABLED", "modbus_polling_enabled", "Modbus Polling Enabled", input_type="checkbox"),
     ConfigField(
         "DSMR",
@@ -351,6 +357,7 @@ def _settings_payload(settings: Settings) -> dict[str, Any]:
         "mqtt_tls_enabled": settings.mqtt_tls_enabled,
         "ha_discovery_enabled": settings.ha_discovery_enabled,
         "ha_discovery_prefix": settings.ha_discovery_prefix,
+        "ha_state_topic_prefix": settings.ha_state_topic_prefix,
         "dsmr_ingest_enabled": settings.dsmr_ingest_enabled,
         "dsmr_mqtt_topic": settings.dsmr_mqtt_topic,
         "dsmr_sample_interval_s": settings.dsmr_sample_interval_s,