ads: add in-content 728x90 banner to gear, technology, forum, authors

These four landing pages already had the site-wide header leaderboard
via <Header />, but no additional in-content banner. Add one 728x90
slot at the bottom of the main column on each, gated on
ADS_728X90.length > 0 and hidden on mobile to match the header
banner's display rules.

  - /gear            — category landing
  - /technology      — category landing
  - /forum           — community landing
  - /authors/[slug]  — author profile

Brings these pages in line with /show-coverage and /about/* which got
the same treatment earlier.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/forum/page.tsx

@@ -3,6 +3,8 @@ import React, { useState, useEffect } from 'react';
 import Link from 'next/link';
 import Header from '@/components/Header';
 import Footer from '@/components/Footer';
+import AdImage from '@/components/AdImage';
+import { ADS_728X90, shuffle } from '@/lib/ads';
 import { createClient } from '@/lib/supabase/client';
 
 interface ForumCategory {
@@ -567,6 +569,13 @@ export default function ForumIndexPage() {
               <li>• No spam, self-promotion, or vendor pitches without disclosure</li>
             </ul>
           </div>
+
+          {/* In-content 728x90 banner */}
+          {ADS_728X90.length > 0 && (
+            <div className="mt-10 hidden md:flex justify-center" aria-label="Advertisement">
+              <AdImage ad={shuffle(ADS_728X90)[0]} page="forum" />
+            </div>
+          )}
         </div>
       </main>
       <Footer />