Align UI palette with Purdue branding

[CunMayday]

Summary

• restyle the app shell with Purdue color tokens for backgrounds, controls, typography, and tables
• update chart shading, threshold line, and slider accent colors to match the Campus and Athletic Gold palette
• switch the bell curve to a plugin-driven render so the threshold slider updates without rebuilding the Chart.js instance

Testing

• not run (not requested)

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https://chatgpt.com/codex/tasks/task_e_68cc87dddb78832c80103710e6c9fb3f