Improve media manager UI: search, delete, drag-and-drop, file meta

Add a richer media manager with filename search, per-file delete with
confirmation, drag-and-drop uploads, copy-URL feedback, file size/type/date
metadata, and server-side upload validation that returns errors to the UI.
Extend the /api/media listing with size and uploaded timestamps.

src/app/admin/media/page.tsx

@@ -1,4 +1,3 @@
-import Link from "next/link";
 import { getTranslations } from "next-intl/server";
 import { AdminMediaGrid } from "@/components/admin/media-grid";
 import { redirect } from "next/navigation";
@@ -14,14 +13,13 @@ export default async function AdminMediaPage() {
 
 	return (
 		<main>
-			<div className="flex justify-end mb-6">
-				<Link
-					href="/admin/articles"
-					className="text-sm theme-text-muted dark:theme-text-muted"
-					style={{ fontSize: "0.88rem" }}
-				>
-					← {t("articles")}
-				</Link>
+			<div className="mb-6">
+				<h1 className="m-0 text-2xl font-extrabold text-[var(--admin-text)]">
+					{t("title")}
+				</h1>
+				<p className="m-0 text-sm text-[var(--admin-text-muted)] mt-1">
+					{t("subtitle")}
+				</p>
 			</div>
 			<AdminMediaGrid />
 		</main>