site: fingerprint the stylesheet

After the progress bar merged, the.corvus.observer served the new HTML with the previous stylesheet: Cloudflare caches CSS at the edge in front of R2 by default and does not cache HTML, so the two went out of step. The stylesheet is now built through Hugo pipes, minified and fingerprinted, so every change gets a new URL and the page always loads the stylesheet it was built with.

Verified with a local build served over HTTP and a headless Chrome screenshot.

Closes #12 (closed)