feat: integrate Sherpa-ONNX local offline speech recognition engine

kouyu_english/lib/core/voice_service.dart

@@ -77,8 +77,12 @@ class VoiceService {
     if (!await recordings.exists()) await recordings.create(recursive: true);
     final timestamp = DateTime.now().microsecondsSinceEpoch;
     await _recorder.start(
-      const RecordConfig(encoder: AudioEncoder.aacLc),
-      path: '${recordings.path}/practice_$timestamp.m4a',
+      const RecordConfig(
+        encoder: AudioEncoder.wav,
+        sampleRate: 16000,
+        numChannels: 1,
+      ),
+      path: '${recordings.path}/practice_$timestamp.wav',
     );
     return true;
   }
@@ -118,7 +122,7 @@ class VoiceService {
     if (!await recordings.exists()) return const [];
     final files = await recordings
         .list()
-        .where((item) => item is File && item.path.endsWith('.m4a'))
+        .where((item) => item is File && (item.path.endsWith('.wav') || item.path.endsWith('.m4a')))
         .cast<File>()
         .toList();
     files.sort((left, right) => right.path.compareTo(left.path));