Plain HTML, CSS and JavaScript modules, with no build step. Hash routes
and relative URLs make it work under any path. The job list is polled:
every 2 seconds while a job runs, and less often otherwise.
Logins follow shelfmark: none, CWA accounts, or a reverse proxy header.
CWA accounts are checked against the OPDS feed, with a lockout after
repeated wrong passwords. An API key works in every mode, and changes
need an X-Requested-With header or the key. URL_BASE serves the app
under a path prefix.