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/accounting.html

@@ -2301,6 +2301,7 @@
     </div>
   </div>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/accounting.js"></script>

pos/templates/catalog.html

@@ -737,6 +737,7 @@
     <button class="banner__dismiss" onclick="document.getElementById('offlineBanner').style.display='none'" aria-label="Cerrar">&times;</button>
   </div>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/catalog.js"></script>

pos/templates/config.html

@@ -1702,6 +1702,44 @@
           </div>
         </div>
 
+        <!-- ===============================================================
+             SECTION 8: MONEDA / CURRENCY
+             =============================================================== -->
+        <div class="settings-section">
+          <div class="settings-section__header">
+            <div class="settings-section__icon">
+              <svg viewBox="0 0 24 24"><line x1="12" y1="1" x2="12" y2="23"/><path d="M17 5H9.5a3.5 3.5 0 0 0 0 7h5a3.5 3.5 0 0 1 0 7H6"/></svg>
+            </div>
+            <div>
+              <div class="settings-section__title" data-i18n="currency_config">Moneda</div>
+              <div class="settings-section__desc">Configura la moneda y tipo de cambio para ventas</div>
+            </div>
+          </div>
+
+          <div class="settings-card" style="padding:var(--space-5);">
+            <div class="form-grid" style="display:grid;grid-template-columns:1fr 1fr;gap:var(--space-4);">
+              <div class="form-group">
+                <label class="form-label" data-i18n="default_currency">Moneda Predeterminada</label>
+                <select class="form-input" id="cfg-currency" style="padding:var(--space-2) var(--space-3);border:1px solid var(--color-border);border-radius:var(--radius-sm,4px);background:var(--color-bg-base);color:var(--color-text-primary);font-size:var(--text-body-sm);">
+                  <option value="MXN">$ MXN — Peso Mexicano</option>
+                  <option value="USD">US$ USD — US Dollar</option>
+                </select>
+              </div>
+              <div class="form-group">
+                <label class="form-label" data-i18n="exchange_rate">Tipo de Cambio USD/MXN</label>
+                <input class="form-input" id="cfg-exchange-rate" type="number" step="0.01" min="0.01" value="17.50"
+                       style="padding:var(--space-2) var(--space-3);border:1px solid var(--color-border);border-radius:var(--radius-sm,4px);background:var(--color-bg-base);color:var(--color-text-primary);font-size:var(--text-body-sm);font-family:var(--font-mono);" />
+              </div>
+            </div>
+            <div style="margin-top:var(--space-4);display:flex;gap:var(--space-3);align-items:center;">
+              <button class="btn btn--primary" id="btn-save-currency" style="padding:var(--space-2) var(--space-4);font-size:var(--text-body-sm);" onclick="Config.saveCurrency()">
+                Guardar Moneda
+              </button>
+              <span id="currency-status" style="font-size:var(--text-caption);color:var(--color-text-muted);"></span>
+            </div>
+          </div>
+        </div>
+
       </div><!-- /content-scroll -->
     </main>
   </div><!-- /app-shell -->
@@ -1876,6 +1914,7 @@
     @keyframes slideUp { from { opacity: 0; transform: translateY(20px); } to { opacity: 1; transform: translateY(0); } }
   </style>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/config.js"></script>

pos/templates/customers.html

@@ -2147,6 +2147,7 @@
     </div>
   </div>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/customers.js"></script>

pos/templates/fleet.html

@@ -962,6 +962,7 @@
   </div>
 </div>
 
+<script src="/pos/static/js/i18n.js"></script>
 <script src="/pos/static/js/app-init.js"></script>
 <script src="/pos/static/js/sidebar.js"></script>
 <script src="/pos/static/js/fleet.js"></script>

pos/templates/inventory.html

@@ -2525,6 +2525,7 @@
     <button class="banner__dismiss" onclick="document.getElementById('offlineBanner').style.display='none'" aria-label="Cerrar">&times;</button>
   </div>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/inventory.js"></script>

pos/templates/invoicing.html

@@ -2723,6 +2723,7 @@
     </div>
   </div>
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/invoicing.js"></script>

pos/templates/reports.html

@@ -1846,6 +1846,7 @@
   </div>
   <!-- End app-shell -->
 
+  <script src="/pos/static/js/i18n.js"></script>
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/reports.js"></script>

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>