Hero falls back to the next image when the lead's won't load

Some sources hotlink-protect their images (e.g. Guardian's i.guim.co.uk → 401),
so a perfectly-enriched lead could still render an imageless hero. The browser is
the only true judge of loadability, so on a hero image error, promote the next
brief item that has an image into the hero slot; the failed lead becomes a text
tile. Resets to the lead on each fresh brief.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/ArticleCard.svelte

@@ -1,5 +1,5 @@
 <script>
-  let { article, onaction, onreplace, ontag, hero = false } = $props();
+  let { article, onaction, onreplace, ontag, onimageerror, hero = false } = $props();
 
   const humanize = (s) => (s || '').replace(/-/g, ' ');
   // Grouping tags are the doorways; cap at 3 so cards stay calm. Fall back to
@@ -49,7 +49,7 @@
   {#if showImage}
     <a class="media" href={safeHref} target="_blank" rel="noopener">
       <img src={article.image_url} alt="" loading="lazy" referrerpolicy="no-referrer"
-           onerror={() => (failed = true)} />
+           onerror={() => { failed = true; onimageerror?.(); }} />
     </a>
   {/if}