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/main.py

@@ -1,4 +1,4 @@
-import asyncio
+import asyncio
 import base64
 import logging
 import os
@@ -9,9 +9,9 @@ import tempfile
 import uuid
 from typing import Optional
 
-from fastapi import FastAPI, HTTPException, Request, Security, File, UploadFile
+from fastapi import FastAPI, HTTPException, Request, Security
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import JSONResponse, Response
+from fastapi.responses import JSONResponse
 from fastapi.security import APIKeyHeader
 from pydantic import BaseModel
 
@@ -133,8 +133,7 @@ def _sanitize_converted_markdown(text: str) -> str:
     value = (text or "").replace("\r\n", "\n").replace("\r", "\n")
     value = IMAGE_MARKDOWN_RE.sub("", value)
     value = IMAGE_HTML_RE.sub("", value)
-    value = re.sub(r"\n{3,}", "\n\n", value)
-    return value.strip()
+    return value
 
 
 def get_client_ip(request: Request) -> str: