feat(mq_editor): spread autodetection, 50% drop-off line, mini-spectrum peak fix

- autodetectSpread(): measures half-power (-3dB) peak width in spectrogram
  to infer spread_above/below (replaces near-zero bezier residual approach)
- 'Auto' button per partial + 'Auto Spread All' toolbar button
- Spread panel inputs now trigger viewer refresh on change
- 50% drop-off dotted reference lines on spread band (selected partial only)
- Mini-spectrum: use max() instead of mean() over bins per pixel column,
  fixing high-frequency amplitude mismatch between original and synthesis

handoff(Gemini): spread autodetection and mini-spectrum fixes done

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