升级 flutter_markdown → flutter_markdown_plus，重写笔记编辑器界面

- flutter_markdown_plus ^1.0.7 替代已弃用的 flutter_markdown
- 新增分屏实时预览：编辑/分屏/预览三模式切换
- 底部现代极简工具栏：浅灰圆角容器，图标按钮分组，智能标题按钮
- 修复光标错位：StrutStyle 统一行高 + TextEditingValue 原子更新

lib/pages/note/note_detail_page.dart

@@ -1,6 +1,6 @@
 import 'dart:io';
 import 'package:flutter/material.dart';
-import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:provider/provider.dart';
 import '../../providers/app_provider.dart';
 import '../../models/data_models.dart';