feed: intersperse mobile-only 300x250 ads every 4 articles

Mobile readers previously saw zero ads above the bottom of the feed
(header leaderboard is hidden md:block, sidebar stack pushes below the
feed at <lg breakpoints). This adds a 300x250 ad block after every 4th
article in <ArticleFeed>, wrapped in md:hidden so tablet/desktop are
untouched (they already get SidebarAdStack to the right of the feed).

- Ad pool: rotateAll('300x250') from src/lib/ads.ts (DB-backed, falls
  back to the 7-ad hardcoded pool)
- Cycles through the pool if the feed is longer than the pool
- Skips the insert after the very last article
- Labelled "Sponsored", divide-y separator inherited from parent

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

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

@@ -1,10 +1,14 @@
 "use client";
-import React, { useState, useRef, useId, useEffect, useCallback } from "react";
+import React, { Fragment, useState, useRef, useId, useEffect, useCallback, useMemo } from "react";
 import AppImage from "@/components/ui/AppImage";
 import SidebarAdStack from "@/components/SidebarAdStack";
+import AdImage from "@/components/AdImage";
+import { rotateAll, type Ad } from "@/lib/ads";
 import { PersonIcon, ChevronLeftIcon, ChevronRightIcon, SearchIcon, CloseIcon } from "@/components/ui/Icons";
 import Link from "next/link";
 
+const MOBILE_AD_EVERY_N = 4;
+
 
 interface ArticleItem {
   title: string;
@@ -86,6 +90,10 @@ export default function ArticleFeed() {
   const sentinelRef = useRef<HTMLDivElement>(null);
   const feedId = useId();
 
+  // Shuffled 300x250 pool for mobile in-feed inserts (md:hidden).
+  // Tablet/desktop already get the SidebarAdStack to the right of the feed.
+  const mobileInfeedAds = useMemo<Ad[]>(() => rotateAll("300x250"), []);
+
   // Load preference from localStorage
   useEffect(() => {
     try {
@@ -550,8 +558,18 @@ export default function ArticleFeed() {
                   : `/articles/${article?.slug}`;
                 const linkProps = isImported ? { target: "_blank", rel: "noopener noreferrer" } : {};
 
+                // Mobile in-feed ad after every Nth article (skip if last item).
+                const isAdSlot =
+                  (i + 1) % MOBILE_AD_EVERY_N === 0 &&
+                  i + 1 < displayedArticles.length &&
+                  mobileInfeedAds.length > 0;
+                const inFeedAd = isAdSlot
+                  ? mobileInfeedAds[Math.floor((i + 1) / MOBILE_AD_EVERY_N - 1) % mobileInfeedAds.length]
+                  : null;
+
                 return (
-                  <article key={i} className="article-card flex gap-3 md:gap-4 py-4 md:py-5 group cursor-pointer rounded-sm px-2 -mx-2">
+                  <Fragment key={`feed-${i}`}>
+                  <article className="article-card flex gap-3 md:gap-4 py-4 md:py-5 group cursor-pointer rounded-sm px-2 -mx-2">
                     <Link
                       href={articleHref}
                       {...linkProps}
@@ -604,6 +622,18 @@ export default function ArticleFeed() {
                       </div>
                     </div>
                   </article>
+                  {inFeedAd && (
+                    <div
+                      className="md:hidden py-5 flex flex-col items-center bg-[#0a0a0a]"
+                      aria-label="Advertisement"
+                    >
+                      <span className="text-[9px] font-mono uppercase tracking-wider text-[#555] mb-2">
+                        Sponsored
+                      </span>
+                      <AdImage ad={inFeedAd} />
+                    </div>
+                  )}
+                  </Fragment>
                 );
               })}
             </div>