feat(transport): Improve socket transport mode selection and localization

lib/core/models/backend_config.dart

@@ -0,0 +1,53 @@
+import 'package:flutter/foundation.dart';
+
+/// Subset of the backend `/api/config` response the app cares about.
+@immutable
+class BackendConfig {
+  const BackendConfig({this.enableWebsocket});
+
+  /// Mirrors `features.enable_websocket` from OpenWebUI.
+  final bool? enableWebsocket;
+
+  /// Returns a copy with updated fields.
+  BackendConfig copyWith({bool? enableWebsocket}) {
+    return BackendConfig(
+      enableWebsocket: enableWebsocket ?? this.enableWebsocket,
+    );
+  }
+
+  /// Whether the backend only allows WebSocket transport.
+  bool get websocketOnly => enableWebsocket == true;
+
+  /// Whether the backend only allows HTTP polling transport.
+  bool get pollingOnly => enableWebsocket == false;
+
+  /// Whether the backend permits choosing WebSocket-only mode.
+  bool get supportsWebsocketOnly => !pollingOnly;
+
+  /// Whether the backend permits choosing polling fallback.
+  bool get supportsPolling => !websocketOnly;
+
+  /// Returns the enforced transport mode derived from backend policy.
+  String? get enforcedTransportMode {
+    if (websocketOnly) return 'ws';
+    if (pollingOnly) return 'polling';
+    return null;
+  }
+
+  Map<String, dynamic> toJson() {
+    return <String, dynamic>{'enable_websocket': enableWebsocket};
+  }
+
+  static BackendConfig fromJson(Map<String, dynamic> json) {
+    bool? enableWebsocket;
+    final features = json['features'];
+    if (features is Map<String, dynamic>) {
+      final value = features['enable_websocket'];
+      if (value is bool) {
+        enableWebsocket = value;
+      }
+    }
+
+    return BackendConfig(enableWebsocket: enableWebsocket);
+  }
+}