Fix shap explainer

[Irozuku]

Summary

Improves the robustness and user experience of explainer plots by adding proper loading and error feedback in the frontend, and fixes SHAP value shape handling in the backend to ensure consistent and correct explainer outputs.

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• frontend/components/ExplainersPlot.tsx:
  Added loading (CircularProgress) and error UI states, improved useEffect logic to fetch plots only when the explainer status is valid, and prevented premature or unnecessary requests.
• frontend/locales/en.json:
  Added a localized error message for explainer failures.
• frontend/locales/es.json:
  Added the Spanish translation for the explainer failure error message.
• backend/explainers/kernel_shap.py:
  Corrected SHAP value shape ordering to (n_instances, n_classes, n_features) using np.transpose instead of swapaxes to ensure compatibility.

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Testing

• Confirmed SHAP outputs have the expected shape and are correctly consumed by the frontend.