feat: implement sensor calibration flow

lib/data/sources/localiser/realtime_data_client.dart

@@ -78,6 +78,12 @@ class RealtimeDataClient {
         .map((msg) => (event: msg.event.value, payload: msg.payload ?? const {}));
   }
 
+  /// Leaves [topic] and removes it from the channel cache.
+  void leaveChannel(String topic) {
+    final ch = _channels.remove(topic);
+    ch?.leave();
+  }
+
   /// Pushes [event] on [topic] and waits for the server reply.
   /// The channel must have been joined first via [channel].
   Future<Map<String, dynamic>> push(