web: serve the stylesheet over a css route

Embed the css directory alongside the templates and serve it with a
FileServerFS mounted on GET /css/. The directory name matches the url
prefix, so the file resolves without an fs.Sub and there is no error
path to handle. Adding js later is the same one line.

Cover the route in the handler tests, including that template sources
stay unreachable and that the href the home page emits resolves to a
file that is actually served.

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