feat: receipt PDF save and print via html2pdf.js

- Save PDF button downloads receipt directly
- Print button opens PDF in new window and triggers print dialog
- Replaces previous window.print() approach that lost styles
- Receipt generated on demand from transaction data, no file storage needed

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",
+    "html2pdf.js": "^0.14.0",
     "jsbarcode": "^3.12.3",
     "jspdf": "^4.2.1",
     "lucide-react": "^1.7.0",