Extract BackgroundConfigEditor as reusable controlled component from
BackgroundEditor. Add background settings section to landing editor.
Render per-landing backgrounds on public purchase pages.
- Extract BackgroundConfigEditor (value/onChange) from BackgroundEditor
- Refactor BackgroundEditor to thin wrapper with API persistence
- Add StaticBackgroundRenderer for prop-based config (public pages)
- Add background_config to landing API types and editor form
- Render animated background on QuickPurchase page
