Fix powder chart y-axis range and clarify planning workflow

[AndrewSazonov]

This PR fixes powder diffraction charts that were being truncated on the
main intensity view by deriving the y-axis from the measured,
calculated, and background intensities with consistent padding, while
preserving the residual plot’s matched scaling.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.52%. Comparing base (b955261) to head (52bc7fc).

Files with missing lines	Patch %	Lines
src/easydiffraction/display/plotters/plotly.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #166      +/-   ##
===========================================
+ Coverage    88.51%   88.52%   +0.01%     
===========================================
  Files          154      154              
  Lines         9435     9442       +7     
  Branches       979      981       +2     
===========================================
+ Hits          8351     8359       +8     
+ Misses         763      762       -1     
  Partials       321      321              

Flag	Coverage Δ
integration	74.41% <7.69%> (-0.05%)	⬇️
unittests	80.44% <92.30%> (+0.02%)	⬆️

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