feat: add CSV invoice export

[nehayadav827]

Summary

Adds CSV export support for invoices using @capacitor/filesystem.

This PR introduces a CSV export flow for the current SocialCalc invoice by parsing the serialized spreadsheet format and exporting it as a properly escaped CSV file for both PWA and hybrid/mobile environments.

Changes

Added

• src/services/csvExportService.ts

  • Parses SocialCalc serialized spreadsheet content
  • Supports escaped values (\c, \n, \b)
  • Handles formula/result cell types (vt, vtf, vtc)
  • Generates properly escaped CSV rows and filenames

Updated

• src/components/Menu/Menu.tsx

  • Added Export as CSV option to the existing ActionSheet
  • Added native/hybrid export support using @capacitor/filesystem
  • Added browser/PWA CSV download fallback
  • Updated toast dismissal handling to avoid reopening the Save As dialog after CSV export actions

• package.json

  • Added @capacitor/filesystem

• package-lock.json

  • Updated dependency lockfile

Acceptance Criteria Addressed

• ✅ Invoices can be exported as CSV
• ✅ Adds Capacitor plugin integration
• ✅ Works across PWA and hybrid/mobile flows
• ✅ Maintains existing Save / Save As behavior

Validation

Verified locally by:

• testing CSV generation from SocialCalc invoice data
• validating escaped commas, quotes, colons, and newline handling
• validating formula/result cell parsing
• running:

npm run build

Build completed successfully.

Screenshot

The screenshot below demonstrates successful CSV generation from serialized SocialCalc invoice data, including:

• parsed invoice rows
• formula result handling
• escaped special characters
• generated CSV filename output