Add export and error/empty states to Custom Report Builder

[Wilfred007]

What

Add report export (CSV, and PDF/Excel where supported) and proper loading/empty/error states to frontend/src/pages/CustomReportBuilder.tsx.

Why

The report builder lets employers select columns and filter rows, but there is no way to export the result, and it lacks loading/empty/error handling. Exporting is the primary reason to build a custom report (accounting, compliance filing). This complements the separate issue that replaces its mock dataset with real data.

Scope

In scope:

• An export action producing at least CSV from the currently built/filtered report.
• Reuse backend export endpoints for PDF/Excel where a report maps to them.
• Loading, empty-result, and error states for the builder.

Out of scope:

• Replacing the mock dataset (tracked separately) — build export against whatever data source is in place, so this lands cleanly after that.
• Scheduled/emailed reports.

Acceptance criteria

• User can export the current report to CSV (download triggers with correct columns/rows).
• Empty and error states render instead of a blank table.
• npm run lint and type build pass.

Technical context

• Page: frontend/src/pages/CustomReportBuilder.tsx.
• Backend export endpoints: backend/src/routes/exportRoutes.ts (/payroll/.../csv, /excel, /receipt/:txHash/pdf).
• Pairs with the 'replace mock dataset in Custom Report Builder' issue.

[waterWang]

Submitted PR #576 to fix this issue. The export button now generates real CSV downloads, and the page has proper loading, empty, and error states.