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/static/js/app-init.js

@@ -40,9 +40,10 @@
 
     var name = employee.name || payload.name || 'Usuario';
     var role = employee.role || payload.role || '';
+    var _t = typeof window.t === 'function' ? window.t : function(k) { return k; };
     var roleLabels = {
-        'owner': 'Dueño', 'admin': 'Administrador', 'cashier': 'Cajero',
-        'warehouse': 'Almacén', 'accountant': 'Contador'
+        'owner': _t('role_owner'), 'admin': _t('role_admin'), 'cashier': _t('role_cashier'),
+        'warehouse': _t('role_warehouse'), 'accountant': _t('role_accountant')
     };
     var roleLabel = roleLabels[role] || role;
     var initials = name.split(' ').map(function(p) { return p[0]; }).join('').toUpperCase().substring(0, 2);