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.
Adds frontend/src/styles.css extracted from the confirmed prototype
(paper/ink/pine palette, Fraunces/Inter/IBM Plex Mono type system,
catalog-tag motif) and makes the minimal JSX adjustments needed for
each component to pick up the styling: wordmark heading, search icon,
upload button icon + modal header/footer, folder chevrons + file
icons, doc-path + save-status in the editor, catalog tags and
no-results treatment in search results.
Also adds .gitignore (venv/node_modules/dist/pycache) and commits the
frontend package-lock.json that was previously untracked.
