HTML error pages for HTML-wanting requests

When we 404 (or other error) in the browser we currently return JSON. This is a terrible mediocre user experience. In general, we should default to JSON responses because Nexus is primarily a JSON API. Fortunately, as you can see in the screenshot, when we get a request from a browser (or more concretely, a request to which we would prefer to respond with HTML) we can tell from the Accept header on the request.

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8

We could instead use the presence or lack of /v1 prefix on the path, since that's the implicit marker for API vs. Console requests, but I think the HTML header is better: we're just doing what they tell us to do. They say they want HTML, give them HTML.

What should the HTML be? The web console currently generates the assets served as console pages, including an index.html which is mostly a dummy container for a React app to run in. The console can easily generate other HTML pages (Vite docs on multiple entry points). This has the advantage of keeping the console responsible for all HTML and CSS, and we could reuse our existing styling.

Start by inspecting the web console's generated index.html and its existing asset generation, then review the Vite multi-page entry-point approach described in the issue. Done means browser requests advertising HTML receive styled HTML error pages, while JSON remains the default for API-oriented requests.