feat(client): GrpcWebClientChannel on web targets via conditional import

Splits channel construction into two files selected at
compile-time via `if (dart.library.html)`:

  channel_factory_io.dart   — native ClientChannel (HTTP/2)
  channel_factory_web.dart  — GrpcWebClientChannel.xhr (HTTP/1.1)

HubClient._channel is now `ClientChannelBase` so both
implementations satisfy the same field type. dart:html-only
imports (`package:grpc/grpc_web.dart`) stay isolated from the
native compile path.

Pairs with the hub-side `fai_grpc_web` adapter crate
(fai/platform 0d07892) — `tonic-web` on the same port as
native gRPC, so one `fai serve` listener handles both
surfaces.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/src/channel_factory_web.dart

@@ -0,0 +1,17 @@
+// Web channel factory — gRPC-Web over HTTP/1.1.
+// Selected on dart:html targets (Flutter web, Dart-for-web) via
+// the conditional import in hub_client.dart. The hub must be
+// running with `tonic-web` enabled — see fai_platform docs
+// architecture/protocol-surfaces.md.
+
+import 'package:grpc/grpc_connection_interface.dart';
+import 'package:grpc/grpc_web.dart';
+
+import 'hub_client.dart' show HubEndpoint;
+
+ClientChannelBase createChannel(HubEndpoint endpoint) {
+  final scheme = endpoint.secure ? 'https' : 'http';
+  return GrpcWebClientChannel.xhr(
+    Uri.parse('$scheme://${endpoint.host}:${endpoint.port}'),
+  );
+}