Export additional images with theme backgrounds

[OldJobobo]

Summary

This PR updates theme export so additional images are included in the exported Omarchy
theme package.

Previously, export only copied the primary wallpaper into backgrounds/.
Now, export also copies images from the editor’s Additional Images section into the
same backgrounds/ directory.

What Changed

• src/AetherWindow.js
  • In _exportTheme(), read additional images from state:
    • themeState.getAdditionalImages()
  • Pass them into themeExporter.setThemeData(...).
• src/services/ThemeExporter.js
  • Extended setThemeData(...) to accept/store additionalImages.
  • Forward additionalImages into configWriter.exportTheme(...).
• src/utils/ConfigWriter.js
  • Extended exportTheme(...) signature with additionalImages = [].
  • Added copy step to place each additional image in:
    • /backgrounds/

Behavior Notes

• Backward compatible: callers that do not pass additional images continue to work ([]
  default).
• Collision behavior is unchanged from existing apply/generate flows:
  • copy target uses basename, so duplicate filenames overwrite the earlier copy. (this is existing behavior, kept to minimize code changes in PR)

Testing

• Verified commit scope is limited to export pipeline files only.
• Verified formatting checks pass in this environment.
• Manual validation expected flow:
  1. Add main wallpaper + additional images in UI.
  2. Export theme.
  3. Confirm exported backgrounds/ contains primary wallpaper and additional images.

Why

This aligns export behavior with apply/generate expectations, so shared/exported themes
preserve the full background set users configured in the UI.

[bjarneo]

Neat and sweet!