fix: address code review comments

- vite.config.ts: fix __dirname for ESM, add 1MB payload limit + JSON validation
- MapViewer.tsx: remove broken window.isTransforming checks, fix callback order
- EditorPage.tsx: derive undoCount from historyManager in handleTransformEnd
- package.json: support Node 20 or 22 in engines

package.json

@@ -44,6 +44,6 @@
     "vite": "^8.0.4"
   },
   "engines": {
-    "node": ">=22.12.0"
+    "node": ">=20.19.0 || >=22.12.0"
   }
 }