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_prompt.py

@@ -28,6 +28,13 @@ def test_prompt_builds_system_and_user():
     assert "MERMAID_CONTEXT" in user_prompt
     assert "PREFIX_ENDS_WITH_NEWLINE" in user_prompt
     assert "SUFFIX_STARTS_WITH_NEWLINE" in user_prompt
+    assert "actual line breaks" in system_prompt
+    assert "start OUTPUT on a new line" in system_prompt
+    assert "Use real line breaks instead of spelled-out escape sequences" in user_prompt
+    assert "make the first character of OUTPUT a real newline" in user_prompt
+    assert "make the last character of OUTPUT a real newline" in user_prompt
+    assert "start output with \\n" not in user_prompt
+    assert "Use single \\n" not in system_prompt
 
 
 def test_cursor_in_fence_detection():