feat: integrate Sherpa-ONNX local offline speech recognition engine

kouyu_english/lib/core/app_state.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ -11,6 +12,7 @@ import 'local_store.dart';
 import 'seed_courses.dart';
 import 'ai_config.dart';
 import 'ai_service.dart';
+import 'sherpa_stt_service.dart';
 
 class AppState extends ChangeNotifier {
   static const _storageKey = 'learning_state_v1';
@@ -270,6 +272,7 @@ class AppState extends ChangeNotifier {
 
   Future<void> load() async {
     try {
+      unawaited(SherpaSttService.instance.initialize());
       final config = await AiConfigFile.loadFromAsset();
       if (config != null) {
         if (config.apiKey != null && config.apiKey!.trim().isNotEmpty) {