feat(manage): add dashboard as manage landing page

- Add Dashboard as first sidebar nav item under Overview section
- Update /manage page to render DashboardOverview instead of redirecting
- Remove redirect to /manage/users

app/(manage)/manage/page.tsx

@@ -1,5 +1,15 @@
-import { redirect } from "next/navigation"
+import { DashboardOverview } from "@/components/manage/dashboard-overview";
 
 export default function ManagePage() {
-  redirect("/manage/users")
+  return (
+    <div className="space-y-6">
+      <div>
+        <h1 className="text-2xl font-bold">Dashboard</h1>
+        <p className="text-sm text-zinc-400">
+          Overview of DeckyVault statistics and moderation queue
+        </p>
+      </div>
+      <DashboardOverview />
+    </div>
+  );
 }