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/pages/QuickPurchase.tsx

@@ -12,6 +12,7 @@ import type {
   LandingPaymentMethod,
   PurchaseRequest,
 } from '../api/landings';
+import { StaticBackgroundRenderer } from '../components/backgrounds/BackgroundRenderer';
 import LanguageSwitcher from '../components/LanguageSwitcher';
 import { cn } from '../lib/utils';
 import { getApiErrorMessage } from '../utils/api-error';
@@ -963,6 +964,7 @@ export default function QuickPurchase() {
 
   return (
     <div className="min-h-dvh overflow-x-hidden bg-dark-950">
+      {config.background_config && <StaticBackgroundRenderer config={config.background_config} />}
       <div className="mx-auto max-w-5xl px-4 py-8 sm:px-6 lg:px-8">
         {/* Language switcher */}
         <div className="mb-4 flex justify-end">