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/services/currency.py

@@ -0,0 +1,55 @@
+"""Multi-currency support for border refaccionarias.
+
+Supports MXN and USD with configurable exchange rate.
+"""
+
+from config import DEFAULT_CURRENCY, EXCHANGE_RATE_USD_MXN
+
+CURRENCIES = {
+    'MXN': {'symbol': '$', 'name': 'Peso Mexicano', 'name_en': 'Mexican Peso', 'decimals': 2},
+    'USD': {'symbol': 'US$', 'name': 'Dolar Estadounidense', 'name_en': 'US Dollar', 'decimals': 2},
+}
+
+
+def convert(amount, from_currency, to_currency, rate=None):
+    """Convert an amount between currencies.
+
+    Args:
+        amount: The numeric amount to convert.
+        from_currency: Source currency code ('MXN' or 'USD').
+        to_currency: Target currency code ('MXN' or 'USD').
+        rate: Optional custom exchange rate (USD->MXN). Defaults to config value.
+
+    Returns:
+        The converted amount, rounded to 2 decimals.
+    """
+    if from_currency == to_currency:
+        return amount
+    if rate is None:
+        rate = EXCHANGE_RATE_USD_MXN
+    if from_currency == 'USD' and to_currency == 'MXN':
+        return round(amount * rate, 2)
+    if from_currency == 'MXN' and to_currency == 'USD':
+        return round(amount / rate, 2)
+    return amount
+
+
+def format_currency(amount, currency='MXN'):
+    """Format an amount with the appropriate currency symbol.
+
+    Args:
+        amount: Numeric value.
+        currency: Currency code.
+
+    Returns:
+        Formatted string like '$1,234.56' or 'US$1,234.56'.
+    """
+    info = CURRENCIES.get(currency, CURRENCIES['MXN'])
+    return f"{info['symbol']}{amount:,.{info['decimals']}f}"
+
+
+def get_currency_info(code=None):
+    """Return currency metadata dict. If code is None, return all."""
+    if code:
+        return CURRENCIES.get(code)
+    return CURRENCIES