add AdSlot component and place banner zones on article pages

Adds <AdSlot zone size /> server component that renders a labeled
placeholder for now (size + zone exposed as data attrs for the future
bb.ad_placements lookup).

Inserted on news/[slug] article template:
- 728×90 leaderboard above the <article>
- 300×600 half-page at top of sidebar
- 300×250 mid-rectangle in sidebar (replaces the old hand-rolled
  placeholder div)
- 728×90 leaderboard below the </article>, before related-articles

Same scheme should be applied to articles/[slug]/page.tsx in a follow-up
once that template's body is finalized.

src/app/news/[slug]/NewsArticleDetailClient.tsx

@@ -4,6 +4,7 @@ import Link from "next/link";
 import AppImage from "@/components/ui/AppImage";
 import Header from "@/components/Header";
 import Footer from "@/components/Footer";
+import AdSlot from "@/components/AdSlot";
 import { createClient } from "@/lib/supabase/client";
 import type { Article } from "@/lib/articles/sampleArticles";
 
@@ -280,6 +281,11 @@ export default function NewsArticleDetailClient({
         </div>
       </div>
 
+      {/* Top leaderboard */}
+      <div className="max-w-container mx-auto px-4 pt-6">
+        <AdSlot zone="article-top" size="728x90" />
+      </div>
+
       {/* Article */}
       <article className="max-w-container mx-auto px-4 py-8 md:py-12">
         <div className="grid grid-cols-1 lg:grid-cols-12 gap-8">
@@ -444,6 +450,9 @@ export default function NewsArticleDetailClient({
 
           {/* Sidebar */}
           <aside className="lg:col-span-4 space-y-6">
+            {/* Sidebar fixed half-page */}
+            <AdSlot zone="article-sidebar-top" size="300x600" />
+
             {/* Related Articles Sidebar */}
             <div className="bg-[#111] border border-[#222] p-5">
               <h3 className="font-body font-bold text-xs text-[#3b82f6] uppercase tracking-widest mb-4 pb-2 border-b border-[#222]">
@@ -491,16 +500,16 @@ export default function NewsArticleDetailClient({
             </div>
 
             {/* Ad placeholder */}
-            <div className="ad-placeholder w-full h-[250px] flex items-center justify-center">
+            <AdSlot zone="article-sidebar-mid" size="300x250" />
-              <div className="text-center">
-                <p className="text-[#444] text-xs font-body">Advertisement</p>
-                <p className="text-[#333] text-[10px] font-body mt-1">300×250</p>
-              </div>
-            </div>
           </aside>
         </div>
       </article>
 
+      {/* Bottom leaderboard */}
+      <div className="max-w-container mx-auto px-4 pb-8">
+        <AdSlot zone="article-bottom" size="728x90" />
+      </div>
+
       {/* Related Articles Carousel */}
       {relatedArticles.length > 0 && (
         <section className="border-t border-[#1a1a1a] bg-[#0d0d0d] py-10">

src/components/AdSlot.tsx

@@ -0,0 +1,49 @@
+import { ReactElement } from "react";
+
+type IabSize = "728x90" | "300x250" | "300x600" | "160x600" | "970x250" | "320x50";
+type Zone =
+  | "article-top"
+  | "article-bottom"
+  | "article-sidebar-top"
+  | "article-sidebar-mid"
+  | "article-inline"
+  | "homepage-top"
+  | "homepage-bottom";
+
+const SIZE_MAP: Record<IabSize, { w: number; h: number }> = {
+  "728x90": { w: 728, h: 90 },
+  "300x250": { w: 300, h: 250 },
+  "300x600": { w: 300, h: 600 },
+  "160x600": { w: 160, h: 600 },
+  "970x250": { w: 970, h: 250 },
+  "320x50": { w: 320, h: 50 },
+};
+
+interface AdSlotProps {
+  zone: Zone;
+  size: IabSize;
+  className?: string;
+}
+
+/**
+ * Banner ad slot. Server component (no client JS).
+ *
+ * Renders a labeled placeholder until populated. The eventual implementation
+ * will query bb.ad_placements (server-side) and render an <a><img></a> for the
+ * winning campaign + record an impression.
+ */
+export default function AdSlot({ zone, size, className = "" }: AdSlotProps): ReactElement {
+  const { w, h } = SIZE_MAP[size];
+  return (
+    <div
+      role="complementary"
+      aria-label={`Advertisement (${size})`}
+      data-ad-zone={zone}
+      data-ad-size={size}
+      className={`mx-auto flex items-center justify-center bg-[#0d0d0d] border border-[#1a1a1a] text-[#444] text-[10px] font-body uppercase tracking-widest ${className}`}
+      style={{ width: w, height: h, maxWidth: "100%" }}
+    >
+      <span>Ad · {size}</span>
+    </div>
+  );
+}