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/config.js

@@ -416,6 +416,68 @@ const Config = (() => {
         });
     }
 
+    // -------------------------------------------------------------------------
+    // Currency
+    // -------------------------------------------------------------------------
+    async function loadCurrency() {
+        try {
+            var res = await fetch(API + '/currency', { headers: headers() });
+            if (!res.ok) return;
+            var d = await res.json();
+            var selCurrency = document.getElementById('cfg-currency');
+            var inpRate = document.getElementById('cfg-exchange-rate');
+            if (selCurrency) selCurrency.value = d.currency || 'MXN';
+            if (inpRate) inpRate.value = d.exchange_rate || 17.5;
+            // Store in localStorage for POS fmt() to pick up
+            localStorage.setItem('pos_currency', d.currency || 'MXN');
+            localStorage.setItem('pos_exchange_rate', d.exchange_rate || 17.5);
+        } catch (e) {
+            console.error('Config.loadCurrency:', e);
+        }
+    }
+
+    async function saveCurrency() {
+        var selCurrency = document.getElementById('cfg-currency');
+        var inpRate = document.getElementById('cfg-exchange-rate');
+        var statusEl = document.getElementById('currency-status');
+        var btn = document.getElementById('btn-save-currency');
+
+        if (!selCurrency || !inpRate) return;
+
+        var currency = selCurrency.value;
+        var rate = parseFloat(inpRate.value);
+
+        if (!rate || rate <= 0) {
+            toast('Tipo de cambio invalido', 'error');
+            return;
+        }
+
+        if (btn) { btn.disabled = true; btn.textContent = 'Guardando...'; }
+
+        try {
+            var res = await fetch(API + '/currency', {
+                method: 'PUT',
+                headers: headers(),
+                body: JSON.stringify({ currency: currency, exchange_rate: rate })
+            });
+            if (!res.ok) {
+                var err = await res.json().catch(function() { return { error: res.statusText }; });
+                throw new Error(err.error || 'Save failed');
+            }
+            localStorage.setItem('pos_currency', currency);
+            localStorage.setItem('pos_exchange_rate', rate);
+            toast('Moneda actualizada');
+            if (statusEl) statusEl.textContent = currency + ' — TC: ' + rate;
+        } catch (e) {
+            toast(e.message, 'error');
+        } finally {
+            if (btn) {
+                btn.disabled = false;
+                btn.textContent = 'Guardar Moneda';
+            }
+        }
+    }
+
     // -------------------------------------------------------------------------
     // Init
     // -------------------------------------------------------------------------
@@ -441,6 +503,7 @@ const Config = (() => {
         loadBranches();
         loadEmployees();
         loadBusiness();
+        loadCurrency();
     }
 
     document.addEventListener('DOMContentLoaded', init);
@@ -448,6 +511,7 @@ const Config = (() => {
     return {
         init, setTheme, selectThemeOption,
         loadBranches, loadEmployees, saveBranch, saveEmployee,
+        loadCurrency, saveCurrency,
         openModal, closeModal
     };
 })();