The CLI content server serves the content directory at the URL root, so
images and other content assets are requested as root-relative paths
(e.g. /adventures/map.png). The dev proxy only forwarded /content, so
those requests fell through to historyApiFallback and the browser got
index.html instead of the asset.
Forward any request that maps to a real file under content/ to the CLI
server, keeping /static/ bundles on the web dev server.