fix: theme preset persistence, page transition flash, and wheel LED jank

- ThemeTab: auto-save presets to server on apply so they persist across navigation
- PageLoader: add bg-dark-950 to prevent transparent flash during lazy-load transitions
- FortuneWheel: replace React state-based LED animation (10 re-renders/sec) with pure CSS @keyframes

src/components/common/PageLoader.tsx

@@ -6,7 +6,7 @@ export default function PageLoader({ variant = 'dark' }: PageLoaderProps) {
   const spinnerColor = variant === 'dark' ? 'border-accent-500' : 'border-blue-500';
 
   return (
-    <div className="flex min-h-[50vh] items-center justify-center">
+    <div className="flex min-h-[50vh] items-center justify-center bg-dark-950">
       <div
         className={`h-10 w-10 border-[3px] ${spinnerColor} animate-spin rounded-full border-t-transparent`}
       />