[Task]: Wallpaper Export Presets

### Summary

Enable exporting gradients directly as wallpapers in common resolutions.

### Milestone

1.4.0 — Animation & Assets

### Details

_No response_

### Checklist

- [ ] Presets for 1080p, 1440p, 4K
- [ ] Export as PNG/JPG with optional scaling
- [ ] Optional batch export mode


### Priority

Low