Add subtle shadow and reduce border intensity on cards

- Add shadow-sm to ServiceCard, BookmarkCard, and Settings card
- Reduce border opacity: slate-100 (light) / stone-700/50 (dark)
- Lighter hover states for a softer look

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/page.tsx

@@ -139,7 +139,7 @@ export default function Home() {
       case 'settings':
         return (
           <div className="max-w-2xl">
-            <div className="bg-white dark:bg-stone-900 rounded-xl border border-slate-200 dark:border-stone-800 p-6">
+            <div className="bg-white dark:bg-stone-900 rounded-xl border border-slate-100 dark:border-stone-700/50 shadow-sm p-6">
               <h2 className="text-lg font-semibold text-slate-900 dark:text-stone-100 mb-6">
                 Appearance
               </h2>