fix: h2 fallback to http/1.1 on handshake failure

lib/utils/http_client.dart

@@ -1,14 +1,53 @@
+import 'dart:io';
+import 'dart:typed_data';
+
 import 'package:dio/dio.dart';
+import 'package:dio/io.dart';
 import 'package:dio_http2_adapter/dio_http2_adapter.dart';
 
+import 'app_logger.dart';
+
 /// Shared HTTP/2 connection pool. All Dio instances that go through
 /// [createHttp2Adapter] reuse the same pool, so connections to the same
 /// host are multiplexed instead of duplicated.
 final _connectionManager = ConnectionManager(idleTimeout: const Duration(seconds: 15));
+final _http1Fallback = IOHttpClientAdapter();
 
-/// Returns an [Http2Adapter] backed by the shared connection pool.
-HttpClientAdapter createHttp2Adapter() => Http2Adapter(_connectionManager);
+/// Returns an [HttpClientAdapter] that tries HTTP/2 first and falls back
+/// to HTTP/1.1 when the server rejects h2 ALPN negotiation.
+HttpClientAdapter createHttp2Adapter() => _Http2WithFallbackAdapter(Http2Adapter(_connectionManager));
 
 /// Shared [Dio] instance for ad-hoc HTTP requests that don't go through
 /// [PlexClient]. Reuses the global HTTP/2 connection pool.
 final httpClient = Dio()..httpClientAdapter = createHttp2Adapter();
+
+class _Http2WithFallbackAdapter implements HttpClientAdapter {
+  _Http2WithFallbackAdapter(this._h2);
+
+  final Http2Adapter _h2;
+
+  static final _http1Hosts = <String>{'plex.tv'};
+
+  @override
+  Future<ResponseBody> fetch(
+    RequestOptions options,
+    Stream<Uint8List>? requestStream,
+    Future<void>? cancelFuture,
+  ) async {
+    if (_http1Hosts.contains(options.uri.host)) {
+      return _http1Fallback.fetch(options, requestStream, cancelFuture);
+    }
+    try {
+      return await _h2.fetch(options, requestStream, cancelFuture);
+    } on HandshakeException {
+      appLogger.d('H2 handshake failed for ${options.uri.host}, falling back to HTTP/1.1');
+      _http1Hosts.add(options.uri.host);
+      return _http1Fallback.fetch(options, requestStream, cancelFuture);
+    }
+  }
+
+  @override
+  void close({bool force = false}) {
+    _h2.close(force: force);
+  }
+}