fix(admin): make admin panel sidebar sticky, matching the feed page

aside now uses h-screen sticky top-0 overflow-y-auto -- same pattern as
the main app Sidebar.tsx -- so it stays in view while scrolling long
admin pages instead of scrolling away with the content.

src/app/p/[secret]/layout.tsx

@@ -72,7 +72,7 @@ export default async function AdminLayout({ children, params }: AdminLayoutProps
   return (
     <div className="flex min-h-screen bg-zinc-950 text-zinc-100">
       {/* Sidebar */}
-      <aside className="w-56 shrink-0 border-r border-zinc-800 flex flex-col py-6 px-4 gap-1">
+      <aside className="w-56 shrink-0 border-r border-zinc-800 flex flex-col py-6 px-4 gap-1 h-screen sticky top-0 overflow-y-auto">
         <p className="text-xs font-semibold text-zinc-500 uppercase tracking-widest px-2 mb-3">
           PawFeed Admin
         </p>