feat: printable receipts with barcode on payment complete

- Receipt component with thermal (80mm) and full-page layout support
- Code 128 barcode from transaction number via JsBarcode
- Store name, address, line items, totals, payment info, barcode
- Print button on sale complete screen (browser print dialog)
- Email button placeholder (disabled, ready for SMTP integration)
- @media print CSS hides everything except receipt content
- Receipt data fetched from GET /transactions/:id/receipt

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

packages/admin/package.json

@@ -25,6 +25,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "date-fns": "^4.1.0",
+    "jsbarcode": "^3.12.3",
     "jspdf": "^4.2.1",
     "lucide-react": "^1.7.0",
     "radix-ui": "^1.4.3",