feat: integrate Sherpa-ONNX local offline speech recognition engine

kouyu_english/lib/core/ai_service.dart

@@ -1,3 +1,4 @@
+import 'sherpa_stt_service.dart';
 import 'dart:io';
 import 'dart:convert';
 
@@ -118,6 +119,12 @@ class AiService {
     required String endpoint,
     required String model,
   }) async {
+    if (filePath.toLowerCase().endsWith('.wav')) {
+      final localText = await SherpaSttService.instance.transcribeWav(filePath);
+      if (localText != null && localText.trim().isNotEmpty) {
+        return localText.trim();
+      }
+    }
     if (provider == AiProviderType.mock) return null;
     final file = File(filePath);
     if (!await file.exists()) return null;