[Feature]: 内置笔记系统

### Problem or Motivation

缺少内置笔记系统：
- 用户无法在平台内快速记录学习笔记，需依赖外部工具，割裂了学习流程。

### Proposed Solution

#### 新增内置笔记系统

- 支持在平台内直接记录笔记
- 支持一键插入白板内容、对话内容至笔记
- 支持导出为 Markdown / PDF 格式到本地
- 新增 AI 笔记功能：基于课程内容自动生成结构化笔记

### Alternatives Considered

_No response_

### Area

UI / UX

### Additional Context

#### Related Issues
This is part of a broader UX improvement proposal. See also:

#37
#36
#34
#33
#32

No files, tests, or entry points are named. Start by reading related issues #37, #36, #34, and #32 to clarify the broader UX scope and identify existing note, whiteboard, conversation, export, and course-content flows; done would cover in-platform notes, content insertion, Markdown/PDF export, and structured AI-generated notes.

Written by the indexing model from the issue text.