meteor3: React 16 -> 18.3 with createRoot

react/react-dom ^18.3.1 (--legacy-peer-deps for the pinned ancient react-*
libs, which still work on 18 via legacy context but gate React 19). Client
entry uses createRoot. Browser-verified on / and /fires; REST smoke
byte-identical (12/12).

package.json

@@ -59,11 +59,11 @@
     "push.js": "^1.0.5",
     "rc-slider": "^8.5.0",
     "rc-tooltip": "^3.7.0",
-    "react": "^16.0.0",
+    "react": "^18.3.1",
     "react-addons-pure-render-mixin": "^15.6.2",
     "react-bootstrap": "^0.31.5",
     "react-confirm": "^0.1.16",
-    "react-dom": "^16.0.0",
+    "react-dom": "^18.3.1",
     "react-helmet": "^5.2.0",
     "react-i18next": "^7.4.0",
     "react-leaflet": "^1.8.0",