feat(frontend): persist selected tab on admin settings page refresh

Store the selected tab of the admin settings page in the URL query string under the tab parameter. Defend against invalid values by validating it against the list of known tabs.

frontend/app/(main)/admin/settings/page.tsx

@@ -1,8 +1,20 @@
 // Copyright 2026 Arctel.net
 // SPDX-License-Identifier: Apache-2.0
 
+import {Suspense} from "react"
+import {Loader2} from "lucide-react"
 import {AdminSettingsPageClient} from "./page-client"
 
 export default function AdminSettingsPage() {
-  return <AdminSettingsPageClient />
+  return (
+    <Suspense
+      fallback={
+        <div className="flex items-center justify-center min-h-[400px]">
+          <Loader2 className="size-6 animate-spin text-primary" />
+        </div>
+      }
+    >
+      <AdminSettingsPageClient />
+    </Suspense>
+  )
 }