Markdown and image previews were rendering blank because
@monaco-editor/react loads its core assets from cdn.jsdelivr.net at
runtime, which the deployment server's network can't reach. Switching
to @uiw/react-codemirror removes the CDN dependency entirely (all
assets ship in the build) and cuts bundle size from ~2.5MB to ~770KB.
Also add proper image/* content-type detection so images render via
<img> instead of being force-decoded as editor text.
