ui-spacer missing after importing a flow that contains ui-spacers when exported

### Current Behavior

It seems as though the ui-spacer is not exported when exporting a flow. I checked this by searching through the flows.json file after exporting a dashboard flow containing multiple ui-spacers and the exported flow file has no `ui-spacer` references.

### Expected Behavior

The `ui-spacer` elements should be exported with the flow so that the dashboard layout is as expected when it is imported into a new Node-RED instance.

### Steps To Reproduce

_No response_

### Environment

- Dashboard version: 1.30.2
- Node-RED version: 4.1.5
- Node.js version: 22.11.0
- npm version: 10.9.0
- Platform/OS: macOS 26.2
- Browser: Chrome 144.0.7559.133

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate