Restyle the admin panel to match AtomCMS's Filament housekeeping look

The admin pages used bare inline-styled tables on the default page
background. Rebuilt the admin shell + added a scoped .admin CSS layer so
the whole panel matches the original AtomCMS Filament look:
- Dark #2d2d44 sidebar (the AtomCMS admin navbar colour) with the nav
  grouped into Overview / Content / Users & access / Economy / Radio /
  System (mirrors the Filament resource groups), an avatar + rank badge
  header, and amber active-link highlighting via a client AdminNavLink
  (usePathname).
- A topbar ("Housekeeping" + signed-in user) over a light content area.
- Carded tables (surface bg, rounded, shadow, tinted header, row hover)
  and a page-title treatment, applied globally so every admin page is
  styled without per-page edits.

Verified in a real authenticated admin session (production server,
amx_test): sidebar renders #2d2d44 sticky, active link amber on dark
text, /admin/users table carded with 7 real rows; dev server serves the
admin rules after a cache refresh. tsc 0, vitest 49/49, next build 0.

src/components/admin/admin-nav-link.tsx

@@ -0,0 +1,21 @@
+"use client";
+
+import Link from "next/link";
+import { usePathname } from "next/navigation";
+
+/**
+ * Sidebar link that highlights when its route is active. "/admin" matches
+ * exactly; every other entry also matches its sub-routes (e.g. /admin/radio is
+ * active on /admin/radio/history) so the current section stays lit.
+ */
+export function AdminNavLink({ href, label }: { href: string; label: string }) {
+  const pathname = usePathname() ?? "";
+  const active =
+    href === "/admin" ? pathname === "/admin" : pathname === href || pathname.startsWith(`${href}/`);
+
+  return (
+    <Link href={href} className={`admin-nav-link${active ? " active" : ""}`}>
+      {label}
+    </Link>
+  );
+}