The dialog now lists the songs it would rebuild, all ticked, each with when
it was last built and whether that was a rebuild, so songs already rebuilt
by hand can be unticked. GET /api/rebuild-all returns that list, so the
page no longer repeats the server's rule for which job stands for a song,
and POST takes the ticked job ids. A selected job that has since been
rebuilt is reported as skipped rather than rebuilt twice.