test: implement comprehensive test suite for multi-language locale switcher i18n #419

- Test next-intl configuration with supported locales (en, es)
- Test middleware locale detection from URL and browser preferences
- Test message file loading from messages/en.json and messages/es.json
- Test LocaleSwitcher component rendering and locale selection
- Test URL path prefix updating on locale changes
- Test string extraction from dashboard, invoice/new, and invoice/[id] pages
- Test number formatting with locale-appropriate separators
- Test currency formatting for invoices with locale conventions
- Test date formatting respecting locale conventions
- Test locale persistence via URL prefix and across navigation
- Test locale persistence across page refreshes
- Test no hardcoded English strings in key pages
- Test invoice status translation and display
- Test LOCALES.md documentation completeness
- Test all CI checks pass with i18n implementation

Closes #419

Author: chukwudiikeh