feat: add configurable animated background for landing pages

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

src/locales/ru.json

@@ -3779,6 +3779,7 @@
       "discountOverridesHint": "Оставьте пустым для использования общей скидки",
       "discountPreview": "Предпросмотр",
       "discountActive": "Скидка",
+      "background": "Анимированный фон",
       "statistics": "Статистика",
       "stats": {
         "title": "Статистика лендинга",