[Feature]: 讨论时白板出现的内容可以保存并导出

### Problem or Motivation

讨论时白板出现的内容可以保存并导出

### Proposed Solution

讨论时白板出现的内容可以保存并导出

### Alternatives Considered

_No response_

### Area

Classroom generation

### Additional Context

_No response_

The issue names only the classroom-generation area and does not identify files, tests, or an entry point. First locate the discussion whiteboard implementation, then clarify how content should be saved, where it should be stored, which export format is required, and how successful save and export should be verified.

Written by the indexing model from the issue text.