feat(playground): add synth parameter tooltips, group drawer with per-param curve/mute controls
- Canvas tooltip follows mouse over synth bars showing param name, value, range, curve
- Group drawer: per-param draggable curve canvas, dual-thumb min/max range slider, mute toggle
- Muted params hide from visualizer (bars redistribute), use fixed value slider instead
- Group curve drag applies relative delta preserving individual param offsets
- Pulsing orange play button when audio engine not initialized (all UI modes)
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