fix: build layout editor API URL from admin root

[Saithej2k]

Summary

• Build layout-editor save requests from the Node-RED admin root and configured dashboard path.
• Stop deriving the admin endpoint from the browser URL, which could duplicate a custom root.
• Cover default, shared, and different admin/node-root configurations.

Closes #2132

Testing

• npm test: 111 passing
• npm run lint: no errors
• npm run build
• git diff --check