- Create beats.js with BeatGrid class: load(), getCalibratedBeats(), getCalibratedOnsets(), snap(), hasBeats getter
- BeatGrid manages calibrationOffset and snapEnabled state
- snap() uses 100ms threshold against calibrated beat positions
- Handles API field variants: beats/beat_times, tempo/tempo_bpm
- Refactor timeline.js: replace beatTimes/calibrationOffset/snapEnabled with this.beatGrid
- Add getters/setters for calibrationOffset and snapEnabled (backward compat with app.js)
- render() now shows DETECTING BEATS... loading indicator and error state
- render() draws beat marks via getCalibratedBeats(), onset ticks via getCalibratedOnsets()
- snapToBeat() delegates to beatGrid.snap()
- app.js chains BPM label update after loadBeats() resolves
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