feat: revamp sensor add flow

lib/data/sources/localiser/realtime_data_client.dart

@@ -56,6 +56,28 @@ class RealtimeDataClient {
         .map((msg) => msg.payload ?? const {});
   }
 
+  /// Like [channel], but includes the Phoenix event name in each emission.
+  Stream<({String event, Map<String, dynamic> payload})> channelMessages(
+    String topic, {
+    Map<String, dynamic> params = const {},
+  }) {
+    final socket = _socket;
+    if (socket == null) throw StateError('RealtimeDataClient not connected');
+
+    final ch = _channels.putIfAbsent(
+      topic,
+      () {
+        final c = socket.addChannel(topic: topic, parameters: params);
+        c.join();
+        return c;
+      },
+    );
+
+    return ch.messages
+        .where((msg) => msg.event.value != 'phx_reply')
+        .map((msg) => (event: msg.event.value, payload: msg.payload ?? const {}));
+  }
+
   /// Pushes [event] on [topic] and waits for the server reply.
   /// The channel must have been joined first via [channel].
   Future<Map<String, dynamic>> push(