Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/app/admin/ads/new/page.tsx

@@ -11,7 +11,9 @@ export default function NewAd() {
         </div>
         <div>
           <h1 className="m-0 text-xl font-extrabold text-[var(--color-text)]">New advertisement</h1>
-          <p className="m-0 text-xs text-[var(--color-text-muted)] mt-0.5"><Link href="/admin/ads">← Advertisements</Link></p>
+          <p className="m-0 text-xs text-[var(--color-text-muted)] mt-0.5">
+            <Link href="/admin/ads">← Advertisements</Link>
+          </p>
         </div>
       </div>
       <form action={createAd} className="admin-card">