Improve Plotly legend controls and modebar visibility

[AndrewSazonov]

This PR improves Plotly chart usability in EasyDiffraction by keeping the modebar visible, adding a legend toggle button only for plots that actually have legends, making the legend background semi-transparent, and ensuring the legend toggle works reliably in both JupyterLab and exported HTML views.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.52%. Comparing base (ca825c4) to head (f7a27f7).

Files with missing lines	Patch %	Lines
src/easydiffraction/display/plotters/plotly.py	88.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #167      +/-   ##
===========================================
- Coverage    88.52%   88.52%   -0.01%     
===========================================
  Files          154      154              
  Lines         9442     9475      +33     
  Branches       981      990       +9     
===========================================
+ Hits          8359     8388      +29     
- Misses         762      764       +2     
- Partials       321      323       +2     

Flag	Coverage Δ
integration	74.21% <20.58%> (-0.20%)	⬇️
unittests	80.47% <88.23%> (+0.02%)	⬆️

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