feat(pos): add real thermal ticket printing via ESC/POS (#21)

- Add thermal_printer.py service generating raw ESC/POS bytes for 58mm/80mm printers
- Add /pos/api/sales/<id>/print endpoint (escpos_raw or browser mode)
- Add printer.js with WebUSB and Web Serial support for direct browser-to-printer
- Add thermal print button in ticket modal with connect/print workflow

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

pos/static/js/pos.js

@@ -26,7 +26,11 @@ const POS = (() => {
     let searchTimeout = null;
     let customerSearchTimeout = null;
 
-    const fmt = (n) => '$' + parseFloat(n || 0).toLocaleString('es-MX', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
+    // Currency-aware formatter: reads pos_currency from localStorage
+    const _posCurrency = localStorage.getItem('pos_currency') || 'MXN';
+    const _currSymbols = { MXN: '$', USD: 'US$' };
+    const _currLocale = _posCurrency === 'USD' ? 'en-US' : 'es-MX';
+    const fmt = (n) => (_currSymbols[_posCurrency] || '$') + parseFloat(n || 0).toLocaleString(_currLocale, { minimumFractionDigits: 2, maximumFractionDigits: 2 });
 
     function headers() {
         return { 'Content-Type': 'application/json', 'Authorization': 'Bearer ' + token };
@@ -940,6 +944,45 @@ const POS = (() => {
         setTimeout(() => { if (area) area.style.display = 'none'; }, 500);
     }
 
+    // ─── Thermal Printing ─────────────────
+
+    async function connectThermal() {
+        if (!window.NexusPrinter) { showToast('Printer module not loaded'); return; }
+        const result = await NexusPrinter.connect();
+        if (result.ok) {
+            showToast('Impresora conectada: ' + (result.name || result.type));
+            _updatePrinterButtons();
+        } else {
+            showToast(result.error || 'No se pudo conectar la impresora');
+        }
+    }
+
+    async function thermalPrint() {
+        if (!window.NexusPrinter || !NexusPrinter.isConnected()) {
+            showToast('Conecte una impresora termica primero');
+            return;
+        }
+        if (!lastSaleId) { showToast('No hay venta para imprimir'); return; }
+        const ok = await NexusPrinter.printSale(lastSaleId);
+        if (ok) {
+            showToast('Ticket enviado a impresora termica');
+        } else {
+            showToast('Error al imprimir. Reconecte la impresora.');
+        }
+    }
+
+    function _updatePrinterButtons() {
+        const connectBtn = document.getElementById('btnConnectPrinter');
+        const thermalBtn = document.getElementById('btnThermalPrint');
+        if (window.NexusPrinter && NexusPrinter.isConnected()) {
+            if (connectBtn) connectBtn.style.display = 'none';
+            if (thermalBtn) thermalBtn.style.display = '';
+        } else {
+            if (connectBtn) connectBtn.style.display = '';
+            if (thermalBtn) thermalBtn.style.display = 'none';
+        }
+    }
+
     // ─── Last Sale ───────────────────────
     async function showLastSale() {
         if (!lastSaleId) { showToast('No hay venta reciente'); return; }
@@ -1078,5 +1121,6 @@ const POS = (() => {
         creditSale, saveQuotation, createLayaway,
         showLastSale, openDrawer,
         showTicket, closeTicketModal, printTicket,
+        connectThermal, thermalPrint,
     };
 })();