125x125 sidebar banner (sized ad serving) + shorts report link

- New 125x125 "square button" banner size; ad serve now filters by width/height,
  and the sidebar slot serves a 125x125 banner (empty until such inventory exists).
- Shorts reel gets a "report this short" link (posts to /api/report-ad).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server.js

@@ -1149,7 +1149,7 @@ const server = http.createServer(async (req, res) => {
     // -- ad engine (spec §8b v1: banners, text, login ads)
     if (p === '/api/ads/slot' && req.method === 'GET') {
       const t = String(u.searchParams.get('type') || 'banner');
-      const ad = await ads.serve(t);
+      const ad = await ads.serve(t, { width: Number(u.searchParams.get('w')) || 0, height: Number(u.searchParams.get('h')) || 0 });
       // login ads: the member clicks "Open Ad" (a real, counted click into a
       // new tab) while the countdown runs on our interstitial page
       if (ad && t === 'login') ad.dwell = ads.rates().loginDwellSeconds || 10;