Guarantee square interactive calibration panel

[uriahf]

What changed

• constrain the upper interactive Plotly calibration panel to a 1:1 predicted/observed scale
• retain the existing shared zoom range and 5% padding behavior
• leave the histogram y-axis unconstrained
• cover both discrete and smooth interactive calibration plots

Why

The full Plotly widget also contains the histogram, so equal widget width/height does not guarantee that the calibration panel itself is square.

Validation

Added a focused Plotly-layout test checking equal x/y ranges, scaleanchor, scaleratio, and that the histogram remains unconstrained.