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_io.dart

@@ -0,0 +1,21 @@
+// Native channel factory — plain gRPC over HTTP/2.
+// Selected on every non-web target (macOS, Linux, Windows, iOS,
+// Android) via the conditional import in hub_client.dart.
+
+import 'package:grpc/grpc.dart';
+import 'package:grpc/grpc_connection_interface.dart';
+
+import 'hub_client.dart' show HubEndpoint;
+
+ClientChannelBase createChannel(HubEndpoint endpoint) {
+  return ClientChannel(
+    endpoint.host,
+    port: endpoint.port,
+    options: ChannelOptions(
+      credentials: endpoint.secure
+          ? const ChannelCredentials.secure()
+          : const ChannelCredentials.insecure(),
+      idleTimeout: const Duration(minutes: 5),
+    ),
+  );
+}