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.

.gitignore

@@ -23,6 +23,8 @@ env/
 .pytest_cache/
 .mypy_cache/
 .ruff_cache/
+htmlcov/
+api_performance_report.md
 
 # Env files
 .env