Export Web Mix gets an 'Upload to web server' box: the mix is built in a
cache staging folder and mirrored with rsync --delete into <path>/<slug>/
on the host set in Preferences → Integrations → Web Server (config.json,
this machine only). An ssh check asks before replacing an existing mix;
a status window follows check, export and upload. The web player gains a
shuffle button (and the S key).
