feat: add filtered invoice CSV export

[Eyanus]

Summary

Adds CSV export for the current filtered invoice working set.

Merchants can now export the invoices currently represented by their selected filters for spreadsheet use, finance review, and downstream reporting.

closes #361

Type of Change

• Feature
• Fix
• Refactor
• Documentation
• Chore

Description

This PR adds end-to-end CSV export support to the invoice list.

Backend

• Added GET /invoices/export.
• Reuses the invoice list filtering state for:
  • Status
  • Asset
  • Due date
  • Search query
• Preserves merchant scoping through the existing merchant-scope mechanism.
• Exports the following invoice fields:
  • Invoice Number
  • Customer
  • Amount
  • Asset
  • Status
  • Due Date
• Limits exports to 10,000 invoices to prevent excessively large exports.
• Escapes CSV values correctly, including embedded quotes and special characters.
• Protects exported values against spreadsheet formula injection.
• Returns appropriate CSV response headers and exported-row metadata.

Frontend

• Added an Export CSV action to the invoice list.
• The export request uses the currently selected invoice filters.
• Downloads the generated CSV using a date-based filename.
• Displays an export-in-progress state.
• Displays an error message when the export fails.

Tests

• Added coverage for successful CSV export.
• Added coverage for unsupported due-date filters.
• Added coverage for the export row limit.
• Existing merchant-isolation and invoice-service tests continue to pass.

Testing Evidence

Backend

Command:

npm test

Result:

• 23/23 test suites passed
• 167/167 tests passed

The invoice service test suite also passes independently.

Frontend

Commands:

npm run lint

Result:

• Passed

npm run build

Result:

• Production build completed successfully.
• All application routes generated successfully.

Screenshots

Not included. The UI change is limited to the invoice-list Export CSV action and its loading/error states.

Checklist

• Code builds successfully
• Tests added/updated where applicable
• Documentation updated if needed
• Linked issue referenced
• Ready for review

[Cedarich]

Kindly resolve conflicts

[Cedarich]

@Eyanus