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),
+    ),
+  );
+}

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}'),
+  );
+}

lib/src/hub_client.dart

@@ -6,8 +6,10 @@
 
 import 'dart:typed_data';
 
-import 'package:grpc/grpc.dart';
+import 'package:grpc/grpc_connection_interface.dart';
 
+import 'channel_factory_io.dart'
+    if (dart.library.html) 'channel_factory_web.dart' as channel_factory;
 import 'generated/fai/v1/common.pb.dart' as pb_common;
 import 'generated/fai/v1/hub.pb.dart' as pb;
 import 'generated/fai/v1/hub.pbgrpc.dart' as grpc_hub;
@@ -78,9 +80,15 @@ typedef SubmitResponse = pb.SubmitResponse;
 
 /// Typed client for the F∆I Hub gRPC surface. Construct once,
 /// reuse for the process lifetime, call [close] on shutdown.
+///
+/// The channel type is picked at compile time:
+///   * native targets → plain HTTP/2 gRPC via [ClientChannel].
+///   * web targets    → HTTP/1.1 gRPC-Web via [GrpcWebClientChannel].
+///                       Requires the hub to be running with the
+///                       `tonic-web` layer enabled.
 class HubClient {
   final HubEndpoint endpoint;
-  final ClientChannel _channel;
+  final ClientChannelBase _channel;
   final grpc_hub.HubClient _hub;
   final grpc_hub.HubAdminClient _admin;
 
@@ -89,17 +97,7 @@ class HubClient {
         _admin = grpc_hub.HubAdminClient(_channel);
 
   factory HubClient({HubEndpoint endpoint = const HubEndpoint()}) {
-    final channel = ClientChannel(
+    return HubClient._(endpoint, channel_factory.createChannel(endpoint));
-      endpoint.host,
-      port: endpoint.port,
-      options: ChannelOptions(
-        credentials: endpoint.secure
-            ? const ChannelCredentials.secure()
-            : const ChannelCredentials.insecure(),
-        idleTimeout: const Duration(minutes: 5),
-      ),
-    );
-    return HubClient._(endpoint, channel);
   }
 
   /// Liveness probe. Returns true when the hub responds with