[frontend] PayrollAnalytics CSV export only covers trend data, not the full dashboard

[Wilfred007]

Description

The CSV export on PayrollAnalytics.tsx only exports the trend chart's underlying data, leaving out other visible breakdowns (e.g. department breakdown, summary totals) that a user reviewing the dashboard would reasonably expect to be included in "export".

Where

frontend/src/pages — PayrollAnalytics.tsx

Acceptance Criteria

• Extend the export to include all currently visible analytics sections (or let the user pick which sections to export)
• Add a test asserting the exported CSV includes the additional sections