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>

public/assets/site.css

@@ -574,3 +574,6 @@ img{max-width:100%}
 .lt-node.deep{color:var(--muted)}
 .lt-node.qualified{border-color:#ffb238;color:#ffd15c;box-shadow:0 0 0 1px rgba(255,177,56,.35)}
 .lt-node.open{border-style:dashed;color:var(--muted);background:transparent}
+
+.sh-report{display:block;text-align:center;margin-top:10px;color:rgba(255,255,255,.45);font-size:12px}
+.sh-report:hover{color:rgba(255,255,255,.8)}