Add model stats view to stats page

[donjguido]

Summary

• Export term_scores in build_api.py so models.json includes full per-term score arrays
• Add toggleable "Model Stats" view to the stats page with:
  • Model selector dropdown and summary cards (total ratings, terms rated, mean score, self-congruence, rater bias)
  • Two donut pie charts: all ratings distribution vs latest score distribution
  • Stacked horizontal bar chart showing per-term score breakdown with min-evaluations filter
  • NxN pairwise congruence matrix with checkbox model selection and color-coded cells (green=similar, red=dissimilar)
• Existing Consensus Analysis view is unchanged and remains the default

Test plan

• Default view loads unchanged (Consensus Analysis)
• Click "Model Stats" toggle — model selector, cards, pies, bar chart, and matrix render
• Switch between models — all sections update
• Adjust min evaluations slider — bar chart filters correctly
• Toggle model checkboxes in congruence matrix — matrix re-renders with selected subset
• Click term names in bar chart — modal opens
• Toggle dark/light theme — all new elements respect theme
• Test on narrow viewport — pie charts and cards stack vertically
• Toggle back to "Consensus Analysis" — original view restored

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