Fix cross-device jam playback sync

[joedickey]

Summary

• Add room-level transport leader tracking so only the playback leader sends cross-device beat-sync messages.
• Send and persist beat-sync position, clock, BPM, and timestamp metadata for late joiners and drift correction.
• Align remote tabs to the leader's Tone.Transport.seconds phase, not just the sequencer step counter.
• Normalize sequencer start/position handling and keep remote playback changes from echoing back as local transport events.
• Add server and room tests for leader persistence, beat-sync metadata forwarding, persisted clock metadata, and room-state join data.

Validation

• git diff --check
• Browser recording: /opt/cursor/artifacts/jam-sync-browser-test.mp4
• Browser sync log: /opt/cursor/artifacts/jam-sync-browser-test.json (28 samples, max sampled clock delta ~0.014s)
• Attempted npm test in server/, but this environment has no npm/node installed.