Add header image for collaborative AI editor post

Generated "twin cursors in a data stream" header image and added
imageWidth/imageHeight to frontmatter.

Made-with: Cursor

Author: samwillis

website/blog/posts/2026-04-06-building-a-collaborative-ai-editor.md

@@ -6,6 +6,8 @@ excerpt: >-
   ...
 authors: [samwillis]
 image: /img/blog/building-a-collaborative-ai-editor/header.jpg
+imageWidth: 1536
+imageHeight: 1024
 tags: [durable-streams, yjs, agents, collaboration, tanstack-ai]
 outline: [2, 3]
 post: true