feat: historical pollution explorer(#892)

[PrathamReddy888]

Pull Request

Description

Closes #892.

Adds an interactive Historical Pollution Explorer that lets users investigate pollution across years, cities, and pollutants in a single view. Example use case — "compare Pune's PM2.5 between January and August" — is now a one-click preset ("Jan – Aug (this year)").

This builds on the existing historicalDataService.js (Open-Meteo fetch + IndexedDB cache) and historicalAggregate.js (hourly → daily rollup) by adding a higher-level aggregation layer (weekly / monthly / yearly resampling), moving-average computation, period-over-period percentage change, and a multi-location comparison overlay.

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Related Issue

Closes #892

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Type of Change

• Bug Fix
• New Feature
• Documentation
• UI/UX Improvement
• Refactoring
• Performance Improvement
• Accessibility

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Changes Made

• New utility module: historicalExplorer.js with pure helpers (resampling, moving average, % change, half-range change, highest periods, CSV generation).
• New service wrapper: historicalExplorerService.js with single-city and multi-city fetch (parallel via Promise.allSettled).
• New component: HistoricalPollutionExplorer.jsx with city multi-select, pollutant selector, view toggle, chart-type toggle, date presets, stats strip, charts with moving-average overlay + safe-limit reference line, multi-city comparison chart, highest-periods list, and CSV export.
• Integration: App.jsx updated with import, nav entry, and render branch.
• Tests: 42 unit tests — 34 for helpers, 8 for component (service + recharts + i18n mocked).

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Testing

• Tested locally
• No console errors
• Existing functionality works as expected

Test Coverage:

• historicalExplorer.test.js: 34 cases covering resampling, moving average, % change, half-range change, highest-periods ranking, CSV generation.
• HistoricalPollutionExplorer.test.jsx: 8 cases covering loading, success render, error state, multi-city comparison, CSV export, pollutant switching, view switching, highest-periods list.

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Screenshots

not applicable

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Checklist

• My code follows the project guidelines.
• I have tested my changes.
• I have updated the documentation if required.
• My changes address the related issue.
• This PR focuses on a single issue.

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Additional Notes

• Architecture decisions:

  • Pure helpers (no React/fetch) for unit-testability.
  • Reuses existing fetch + cache + aggregate pipeline.
  • Promise.allSettled ensures multi-city fetch resilience.
  • SAFE_LIMITS reused for reference lines.
  • Two CSV schemas: daily (existing format) vs aggregated (new).
  • Moving average overlay only on daily view.

• Out of scope / follow-ups:

  • Add i18n keys for new strings.
  • "Save view as shareable URL" feature.
  • Per-city CSV export in comparison mode.

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Thank You for Your Contribution! 🎉

Hi @PrathamReddy888,

Thank you for opening this Pull Request and contributing to our project. We truly appreciate your efforts.

Please make sure that:

• Your code follows the project's guidelines.
• You have linked the appropriate issue (if applicable).
• Screenshots are added for UI/UX changes.
• Your PR is ready for review.

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[Aditya8369]

@PrathamReddy888 change the branch name and push again