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/tts_service.dart

@@ -6,6 +6,7 @@ import 'package:ai_chat_assistant/utils/common_util.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_tts/flutter_tts.dart';
 
+@Deprecated('Use TtsService interface and its implementations instead')
 class LocalTtsService {
   final FlutterTts _flutterTts = FlutterTts();
   bool _isPlaying = false;