remove rocket.new leaderboard banner; re-add admin nav guard

- src/app/home-page/components/LeaderboardAd.tsx: drop the hardcoded
  img.rocket.new image (rocket_gen_img_1bebfd3c7) at the top of the
  homepage. Replaced with <AdSlot zone='homepage-top' size='728x90' />
  to keep the slot for the eventual ad-server wiring while removing the
  rocket.new dependency entirely.
- src/components/Header.tsx: re-add the isAdmin gate that was reverted
  in 0d51dad. Reads bb.user_profiles.role and only renders the desktop
  admin nav block (WP Import, Articles, Editorial, Users, Analytics,
  Audit Log, Notifications, Newsletter, Forum Seed, Company Tracker,
  Show Calendar, AI Console, Banned Terms, Dashboard) and the mobile WP
  Import link when role is in (administrator, admin, super_admin,
  editor, support). 'support' is admin-tier per the new RBAC matrix.
  Page-level auth checks on each /admin/* route remain authoritative;
  this only suppresses menu visibility for non-admin/anonymous.

src/app/home-page/components/LeaderboardAd.tsx

@@ -1,22 +1,12 @@
 import React from "react";
-import AppImage from "@/components/ui/AppImage";
+import AdSlot from "@/components/AdSlot";
 
-// DO NOT OVERRIDE — Leaderboard ad placeholder layout
 export default function LeaderboardAd() {
   return (
     <div className="bg-ice-blue border-b border-border py-3">
       <div className="max-w-container mx-auto px-4 flex justify-center">
-        {/* Leaderboard ad unit — 728x90 — stub links to advertise page */}
-        <a href="/advertise" aria-label="Advertisement — click to learn about advertising on BroadcastBeat" className="block">
-          <AppImage
-            src="https://img.rocket.new/generatedImages/rocket_gen_img_1bebfd3c7-1765090260606.png"
-            alt="BroadcastBeat sponsor advertisement banner 728x90"
-            width={728}
-            height={90}
-            className="max-w-full h-auto" />
-          
-        </a>
+        <AdSlot zone="homepage-top" size="728x90" />
       </div>
-    </div>);
-
+    </div>
+  );
 }