fix: resolve 'full black' page — unstuck MultiStepLoader overlay, add dark-mode depth

Root cause of the all-black view: the gallery demo hardcoded
<MultiStepLoader loading /> (loading=true forever), which renders a
fixed inset-0 z-[100] backdrop-blur overlay over the entire viewport.
Replace with an interactive demo (button triggers loader, auto-resets
after 4s).

Also soften flat-black dark backgrounds:
- /ui page: dark:bg-black -> dark:bg-zinc-950 + subtle top radial gradient
- home hero: dark:bg-black -> dark:bg-zinc-950 + white 6% radial gradient

components/home-hero.tsx

@@ -9,7 +9,7 @@ import { Spotlight } from "@/components/ui/spotlight";
 
 export function HomeHero() {
   return (
-    <div className="relative flex w-full flex-1 flex-col items-center justify-center overflow-hidden bg-zinc-50 antialiased dark:bg-black">
+    <div className="relative flex w-full flex-1 flex-col items-center justify-center overflow-hidden bg-zinc-50 antialiased dark:bg-zinc-950 dark:bg-[radial-gradient(ellipse_at_top,rgba(255,255,255,0.06),transparent_60%)]">
       <div className="pointer-events-none absolute inset-0 z-0">
         <Spotlight className="-top-40 left-0 md:-top-20 md:left-60" fill="white" />
       </div>