perf: prefetch background chunk on page load from localStorage cache

- Add prefetchBackground() to registry — triggers dynamic import
  without creating React component
- Call prefetch at module init in BackgroundRenderer using cached
  config from localStorage, before React renders
- Background JS chunk starts downloading immediately instead of
  waiting for Suspense to trigger lazy() import

src/components/backgrounds/BackgroundRenderer.tsx

@@ -3,7 +3,7 @@ import { useQuery } from '@tanstack/react-query';
 import { brandingApi } from '@/api/branding';
 import type { AnimationConfig, BackgroundType } from '@/components/ui/backgrounds/types';
 import { DEFAULT_ANIMATION_CONFIG } from '@/components/ui/backgrounds/types';
-import { backgroundComponents } from '@/components/ui/backgrounds/registry';
+import { backgroundComponents, prefetchBackground } from '@/components/ui/backgrounds/registry';
 
 const ANIMATION_CACHE_KEY = 'cabinet_animation_config';
 
@@ -16,6 +16,14 @@ function getCachedConfig(): AnimationConfig | null {
   }
 }
 
+// Prefetch the background JS chunk immediately based on localStorage cache.
+// This starts the download before React even renders, so by the time
+// Suspense needs the component, the chunk is already loaded.
+const cachedConfig = getCachedConfig();
+if (cachedConfig?.enabled && cachedConfig.type && cachedConfig.type !== 'none') {
+  prefetchBackground(cachedConfig.type);
+}
+
 function setCachedConfig(config: AnimationConfig) {
   try {
     localStorage.setItem(ANIMATION_CACHE_KEY, JSON.stringify(config));