feat: enhance Milkdown editor and file system functionality

- Normalize line endings in Markdown export for DOCX files.
- Improve selection serialization to Markdown with better handling of empty documents.
- Add a new `updateFile` function to the file system for updating file properties.
- Introduce video transcoding capabilities using FFmpeg, supporting various video formats.
- Update AGENTS.md for clearer plugin structure and responsibilities.
- Add scoped styles for TreeNodeItem component to improve UI consistency.
- Implement cross-origin isolation headers in Vite configuration for enhanced security.
- Remove obsolete test_cross.py file.

backend/tests/test_tts_asr_unit.py

@@ -12,8 +12,7 @@ TTS/ASR模块单元测试
 import os
 import sys
 import unittest
-from unittest.mock import Mock, MagicMock, patch
-import tempfile
+from unittest.mock import patch
 import numpy as np
 
 # 确保可以导入backend和tts_asr模块