feat(sysadmin): Part 4 — QR code section in SiteDetailPage

- Install qrcode.react in sysadmin_panel
- Add QR Codes section above Remote Managers with two codes side by side:
  Menu QR (https://yourdomain.com/menu/{site_id}) and
  Order QR (https://yourdomain.com/menu/{site_id}/order)
- Each card shows the URL as copyable text and a Download PNG button
- Uses site.site_id as the slug (no slug field exists on the Site model)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sysadmin_panel/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "axios": "^1.7.9",
+    "qrcode.react": "^4.2.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-hot-toast": "^2.4.1",