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_main_endpoints.py

@@ -1,7 +1,7 @@
 import os
 import sys
 import base64
-import asyncio
+import types
 import pytest
 from unittest.mock import MagicMock
 from fastapi.testclient import TestClient
@@ -11,6 +11,10 @@ BACKEND_DIR = os.path.abspath(os.path.join(CURRENT_DIR, ".."))
 if BACKEND_DIR not in sys.path:
     sys.path.insert(0, BACKEND_DIR)
 
+fake_tts_asr = types.ModuleType("tts_asr")
+fake_tts_asr.register_tts_asr_routes = lambda app: None
+sys.modules.setdefault("tts_asr", fake_tts_asr)
+
 import main  # type: ignore
 
 API_KEY = main.API_KEY
@@ -61,13 +65,13 @@ def test_sanitize_markdown_strips_img_tag():
     assert "<img" not in main._sanitize_converted_markdown("<img src='x.png'/>")
 
 
-def test_sanitize_markdown_collapse_newlines():
-    assert main._sanitize_converted_markdown("a\n\n\nb\n\n\n\nc") == "a\n\nb\n\nc"
+def test_sanitize_markdown_preserves_extra_newlines():
+    assert main._sanitize_converted_markdown("a\n\n\nb\n\n\n\nc") == "a\n\n\nb\n\n\n\nc"
 
 
 def test_sanitize_markdown_normalize_crlf():
     result = main._sanitize_converted_markdown("line1\r\nline2\r\n")
-    assert "line1\nline2" in result
+    assert result == "line1\nline2\n"
     assert "\r" not in result