Refactor chart view into modal

[hjyssg]

Summary

• replace the old ChartPage route with a reusable ChartModal component that renders inside the app shell
• compute chart aggregates inside ExplorerPage, pass them up through App, and disable the chart entry when data is unavailable
• remove the unused chart info API from the frontend/backend and style the new modal experience

Testing

• npm test (fails: mocha not found in current environment)

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https://chatgpt.com/codex/tasks/task_e_68d85ded22f48325a5e22d745d510fea