feat: section open/close animations via ViewTransition and @starting-style

Enable experimental.viewTransition in Next.js config. Wrap active section
in ViewTransitionWrapper so the browser cross-fades between sections on
navigation. Replace animate-fadeIn keyframe with @starting-style + CSS
transition for the initial render enter animation.

next.config.ts

@@ -8,6 +8,9 @@ const isExport = process.env.STATIC_EXPORT === 'true';
 const nextConfig: NextConfig = {
   ...(isExport ? { output: 'export' } : {}),
   images: { unoptimized: true },
+  experimental: {
+    viewTransition: true,
+  },
 };
 
 export default nextConfig;