mgr/dashboard: add text/javascript to gzip mime types
CherryPy's serve_file() uses Python's mimetypes module which returns
text/javascript for .js files (RFC 9239), but the gzip config only
listed application/javascript. This meant the main Angular bundle
was being served uncompressed. With Cheroot write timeouts,
the uncompressed transfer times out over slower networks, causing
ERR_CONTENT_LENGTH_MISMATCH in the browser.