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.

src/plugins/docBlockPlugin.ts

@@ -16,6 +16,9 @@ import {
   stripDocBlockMarkdown,
 } from '../utils/docBlock.js'
 
+const FALLBACK_BLOCK_SEPARATOR = '\n\n'
+const FALLBACK_LEAF_TEXT = '\n'
+
 function serializeRangeToMarkdown(
   doc: ProseNode,
   from: number,
@@ -27,7 +30,7 @@ function serializeRangeToMarkdown(
   const slice = doc.slice(from, to)
   if (slice.content.size <= 0) return ''
   const sliceDoc = schema.topNodeType.createAndFill(undefined, slice.content)
-  return sliceDoc ? serializer(sliceDoc) : doc.textBetween(from, to, '\n', '\n')
+  return sliceDoc ? serializer(sliceDoc) : doc.textBetween(from, to, FALLBACK_BLOCK_SEPARATOR, FALLBACK_LEAF_TEXT)
 }
 
 function buildDocContext(doc: ProseNode, excludePos?: number) {