Source: git.varasys.io/VARASYS/metronome
This is a single-page app — save this page (Ctrl/⌘+S) and open the file to run it fully offline, no server needed. One catch when running from a local file://: it won't auto-save your set list between sessions, so export a backup (set-list ⋯ menu → Export all) to keep your work.