Sidebar overflowed horizontally on long file/folder names, which broke
vertical scrolling too — locked width with min/max-width plus
overflow-x: hidden, and truncate folder names with ellipsis.
Documents now open in a rendered preview (marked + DOMPurify) by
default, with an 编辑/预览 toggle to switch to the CodeMirror source view.
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.
