fix: require configured mqtt iata before broker connect

docs/custom-cli.md

@@ -14,6 +14,7 @@ These commands are available on `*_repeater_mqtt` firmware targets.
 - `get mqtt.statuscfg`: shows whether periodic status messages are enabled as a simple `on` or `off` value. Most users can just use `get mqtt.status`.
 - `get mqtt.iata`: shows the IATA/location code used in MQTT topics.
 - `set mqtt.iata <code>`: sets the IATA/location code, for example `MEL`.
+- `set mqtt.iata UNSET`: marks MQTT IATA as not configured yet. While it is `UNSET`, enabled MQTT brokers stay disconnected until a real code is saved.
 
 ### MQTT Identity
 
@@ -43,6 +44,12 @@ These commands are available on `*_repeater_mqtt` firmware targets.
 - `get mqtt.letsmesh-us`
 - `set mqtt.letsmesh-us on|off`
 
+Notes:
+
+- new repeater MQTT installs default `mqtt.iata` to `UNSET`
+- `letsmesh-eu` and `letsmesh-us` remain off by default unless already configured in saved prefs
+- if `mqtt.iata` is `UNSET`, `eastmesh-au`, `letsmesh-eu`, and `letsmesh-us` will not connect even if they are toggled on
+
 Legacy dotted aliases are also accepted:
 
 - `mqtt.eastmesh.au`