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/services/thermal_printer.py

@@ -0,0 +1,119 @@
+"""ESC/POS thermal printer commands for 58mm and 80mm printers.
+
+Generates raw ESC/POS byte commands that can be sent to a thermal printer
+via USB, serial, or network connection.
+"""
+
+# ESC/POS command constants
+ESC = b'\x1b'
+GS = b'\x1d'
+INIT = ESC + b'@'           # Initialize printer
+CUT = GS + b'V' + b'\x00'  # Full cut
+PARTIAL_CUT = GS + b'V' + b'\x01'
+FEED = ESC + b'd'           # Feed N lines
+ALIGN_LEFT = ESC + b'a' + b'\x00'
+ALIGN_CENTER = ESC + b'a' + b'\x01'
+ALIGN_RIGHT = ESC + b'a' + b'\x02'
+BOLD_ON = ESC + b'E' + b'\x01'
+BOLD_OFF = ESC + b'E' + b'\x00'
+DOUBLE_HEIGHT = ESC + b'!' + b'\x10'
+NORMAL_SIZE = ESC + b'!' + b'\x00'
+LARGE_SIZE = ESC + b'!' + b'\x30'  # Double width + double height
+
+
+def generate_ticket(sale_data, business_info, width=80):
+    """Generate ESC/POS bytes for a sale ticket.
+
+    Args:
+        sale_data: dict with sale info (items, totals, payment, folio)
+        business_info: dict with business name, RFC, address
+        width: 58 or 80 (mm)
+
+    Returns: bytes ready to send to printer
+    """
+    chars = 32 if width == 58 else 48  # characters per line
+    buf = bytearray()
+    buf += INIT
+
+    # Header: business name (centered, bold, large)
+    buf += ALIGN_CENTER
+    buf += LARGE_SIZE
+    buf += (business_info.get('name', 'NEXUS POS') + '\n').encode('cp437', errors='replace')
+    buf += NORMAL_SIZE
+    buf += (business_info.get('rfc', '') + '\n').encode('cp437', errors='replace')
+    buf += (business_info.get('address', '') + '\n').encode('cp437', errors='replace')
+    buf += b'\n'
+
+    # Folio + date
+    buf += ALIGN_LEFT
+    buf += BOLD_ON
+    folio = sale_data.get('folio', 'N/A')
+    date = sale_data.get('date', '')
+    buf += f'Folio: {folio}\n'.encode('cp437', errors='replace')
+    buf += BOLD_OFF
+    buf += f'Fecha: {date}\n'.encode('cp437', errors='replace')
+    buf += f'Cajero: {sale_data.get("employee", "")}\n'.encode('cp437', errors='replace')
+    if sale_data.get('customer'):
+        buf += f'Cliente: {sale_data["customer"]}\n'.encode('cp437', errors='replace')
+    buf += ('-' * chars + '\n').encode()
+
+    # Column header
+    buf += BOLD_ON
+    hdr = _format_line('Cant  Descripcion', 'Importe', chars)
+    buf += (hdr + '\n').encode('cp437', errors='replace')
+    buf += BOLD_OFF
+    buf += ('-' * chars + '\n').encode()
+
+    # Items
+    for item in sale_data.get('items', []):
+        name = item.get('name', '')[:chars - 10]
+        qty = item.get('quantity', 1)
+        subtotal = item.get('subtotal', 0)
+        buf += f'{qty}x {name}\n'.encode('cp437', errors='replace')
+        buf += ALIGN_RIGHT
+        buf += f'${subtotal:,.2f}\n'.encode('cp437', errors='replace')
+        buf += ALIGN_LEFT
+
+    buf += ('-' * chars + '\n').encode()
+
+    # Totals
+    buf += ALIGN_RIGHT
+    buf += _total_line('Subtotal:', sale_data.get('subtotal', 0), chars).encode('cp437', errors='replace')
+    if sale_data.get('discount_total', 0) > 0:
+        buf += _total_line('Descuento:', -sale_data['discount_total'], chars).encode('cp437', errors='replace')
+    buf += _total_line('IVA 16%:', sale_data.get('tax_total', 0), chars).encode('cp437', errors='replace')
+    buf += BOLD_ON + DOUBLE_HEIGHT
+    buf += _total_line('TOTAL:', sale_data.get('total', 0), chars).encode('cp437', errors='replace')
+    buf += NORMAL_SIZE + BOLD_OFF
+
+    # Payment
+    buf += ALIGN_LEFT
+    buf += f'\nPago: {sale_data.get("payment_method", "Efectivo")}\n'.encode('cp437', errors='replace')
+    if sale_data.get('amount_paid'):
+        buf += f'Recibido: ${sale_data["amount_paid"]:,.2f}\n'.encode('cp437', errors='replace')
+    if sale_data.get('change_given'):
+        buf += f'Cambio: ${sale_data["change_given"]:,.2f}\n'.encode('cp437', errors='replace')
+
+    # Footer
+    buf += b'\n'
+    buf += ALIGN_CENTER
+    buf += 'Gracias por su compra!\n'.encode('cp437', errors='replace')
+    buf += 'Nexus Autoparts POS\n'.encode('cp437', errors='replace')
+    buf += b'\n\n\n'
+    buf += PARTIAL_CUT
+
+    return bytes(buf)
+
+
+def _format_line(left, right, width):
+    """Pad a left-right line to fill the ticket width."""
+    space = width - len(left) - len(right)
+    if space < 1:
+        space = 1
+    return left + ' ' * space + right
+
+
+def _total_line(label, amount, width):
+    """Format a totals line like 'Subtotal:        $1,234.56'."""
+    val = f'${abs(amount):,.2f}' if amount >= 0 else f'-${abs(amount):,.2f}'
+    return _format_line(label, val, width) + '\n'