feat: add payroll analytics API endpoint and interactive chart UI (#7…

[leojay-net]

• backend: add analyticsController with payroll trends, currency breakdown, payment metrics, department breakdown, and summary queries via PostgreSQL
• backend: add analyticsRoutes with Swagger JSDoc and register under /api/v1/analytics
• frontend: wire PayrollAnalytics to real API with graceful mock fallback
• frontend: add chart type toggle (line/area), quick date presets (3M/6M/YTD/1Y), department breakdown horizontal bar chart, CSV export, and refresh button
• frontend: add PayrollAnalytics unit tests covering rendering, presets, chart toggle, and API fallback behaviour

Closes #751
Closes #645
Closes #768
Closes #716

Pull Request

Summary

What Changed

Checklist

• I linked the relevant issue(s) in the summary.
• I added or updated tests for the change.
• I ran the relevant test suite locally.
• I updated documentation where needed, or explained why it was not needed.
• If this change touches the UI, I verified responsive behavior and accessibility.
• I included screenshots, logs, or other proof when they help review.

Testing

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Notes

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