feat: 新增ai_chat_core，将core 和 widget 分离（未完成）

迁移了 models enums， utils， http 封装，还有一些extensions；service 只迁移了 sse service

lib/services/voice_recognition_service.dart

@@ -2,6 +2,7 @@ import 'package:http/http.dart' as http;
 import 'dart:convert';
 import 'package:http_parser/http_parser.dart';
 
+@Deprecated('VoiceRecognitionService is deprecated, please use the new implementation if available.')
 class VoiceRecognitionService {
   Future<String?> recognizeSpeech(List<int> audioBytes,
       {String lang = 'cn'}) async {