DJ Visualizer Pi: standalone Node+WebSocket server with mobile PWA remote

New separate project (the Electron Mac app stays untouched). Reuses the visual
engine (WebGL shaders, effect catalog, Web Audio) verbatim and replaces Electron
IPC with a Node HTTP+WebSocket server:

- server.js: serves the kiosk output page + mobile control PWA, relays the same
  ctl/rpt JSON protocol over WebSocket, plus REST for media library/upload,
  JSON state (pads), waveform peaks and MP4 recording (ffmpeg).
- public/js/net.js: window.djv shim over WebSocket+HTTP so output.js runs intact.
- public/index.html + control.js: touch-first mobile remote (effects, media/queue,
  ticker, pads, audio/EQ, fullscreen + recording).
- PWA (manifest + service worker + generated icons).
- scripts/: Chromium kiosk launcher + install-pi.sh (systemd units for Pi 5).

Verified: REST endpoints, WebSocket relay both directions, control PWA loads the
2958-effect catalog and renders. (Headless WebGL output not testable without GPU.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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