WebSocket (/ws):
- Replaces 2s HTTP polling with persistent WS connection
- Server pushes status updates every 500ms to all clients
- Server pushes FFT spectrum frames at ~30fps
- Bidirectional: client sends commands as JSON ({cmd, delta, level, ...})
- Auto-reconnect on disconnect (3s backoff)
- Status pushed immediately on connect

Visualisation (internal/viz/capture.go):
- WASAPI loopback capture via IAudioClient (same COM approach as volume)
- Captures whatever is playing through the default render device
- 2048-sample Hanning-windowed FFT (pure Go, no deps)
- 64 log-spaced bars, 40Hz-20kHz
- Fast attack / slow decay smoothing per bar

Canvas renderer (app.js):
- requestAnimationFrame loop, DPR-aware resize
- Green->yellow->red HSL gradient by amplitude
- Peak-hold indicators with 1.2%/frame decay
- Graceful: canvas stays dark if no viz data (Winamp paused/stopped)

Architecture:
- internal/server/hub.go: gorilla/websocket hub (register/broadcast/unregister)
- internal/server/server.go: full //go:build windows, REST API kept for debug
- frontend uses WS for all commands, REST only for killist list fetch

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>