feat(pos): add multi-language i18n (#37) and multi-currency USD/MXN (#38)

- i18n.js with 130+ translation keys for es/en, loaded in all 11 templates
- sidebar.js uses t() for all nav labels, adds MX/US language toggle
- app-init.js role labels use i18n
- currency.py service with convert() and format_currency()
- config.py adds DEFAULT_CURRENCY and EXCHANGE_RATE_USD_MXN settings
- config_bp.py adds GET/PUT /pos/api/config/currency endpoints
- config.html adds currency/exchange-rate section (Section 8)
- config.js adds loadCurrency/saveCurrency with localStorage sync
- pos.js fmt() reads pos_currency from localStorage for USD/MXN display

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/templates/whatsapp.html

@@ -531,6 +531,7 @@ function posLogout(){localStorage.removeItem('pos_token');window.location.href='
 </script>
 
 <!-- Sidebar -->
+<script src="/pos/static/js/i18n.js"></script>
 <script src="/pos/static/js/whatsapp.js"></script>
 <script src="/pos/static/js/sidebar.js"></script>