Fix step arc overlays and key label visibility

Created by: jedmund

Summary

• Fix broken overlay width propagation in KeyboardContainer (pass measured width directly instead of PreferenceKey)
• Increase step arc visibility: thicker strokes, larger labels, more tier spacing
• Position arc labels relative to visual bezier apex for consistent spacing across all arcs
• Expand arc tap targets to include label text area
• Remove long press gesture that was firing a chord before the tap interval played
• Darken white key labels from Color.secondary to Color(white: 0.45)

Test plan

• Open any keyboard tool playground, toggle step arcs on
• Arcs render correctly over keys with visible strokes and readable labels
• Tap any arc label (W, H) or arc stroke to play the interval
• Each arc responds independently (no ghost triggers from overlapping hit areas)
• White key note labels (C4, D4, etc.) are readable against the white keys
• Adjust arc count 1-7 and verify stacking looks correct