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.

package.json

@@ -12,6 +12,10 @@
   },
   "dependencies": {
     "@blocknote/xl-docx-exporter": "^0.47.3",
+    "@ffmpeg/core": "^0.12.10",
+    "@ffmpeg/core-mt": "^0.12.10",
+    "@ffmpeg/ffmpeg": "^0.12.15",
+    "@ffmpeg/util": "^0.12.2",
     "@milkdown/core": "^7.18.0",
     "@milkdown/crepe": "^7.18.0",
     "@milkdown/kit": "^7.18.0",