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-lock.json

@@ -9,6 +9,7 @@
       "version": "0.0.0",
       "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",
@@ -2472,6 +2473,15 @@
         "node": ">=10"
       }
     },
+    "node_modules/qrcode.react": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/qrcode.react/-/qrcode.react-4.2.0.tgz",
+      "integrity": "sha512-QpgqWi8rD9DsS9EP3z7BT+5lY5SFhsqGjpgW5DY/i3mK4M9DTBNz3ErMi8BWYEfI3L0d8GIbGmcdFAS1uIRGjA==",
+      "license": "ISC",
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/queue-microtask": {
       "version": "1.2.3",
       "resolved": "https://registry.npmjs.org/queue-microtask/-/queue-microtask-1.2.3.tgz",