Fix ranking card split: replace nested <a> Source link with button in local book cards

An <a> element nested inside the card's outer <a> is invalid HTML — browsers
auto-close the outer anchor before the inner one, causing the card to be split
across two grid cells visually. Replace with a <button> that calls window.open()
with event.preventDefault/stopPropagation to avoid interfering with card navigation.

scraper/internal/server/ui.go

@@ -645,13 +645,13 @@ const rankingTmpl = `
         </div>
         {{if .SourceURL}}
         <div class="mt-2">
-          <a href="{{.SourceURL}}" target="_blank" rel="noopener noreferrer"
-             class="inline-flex items-center gap-1 text-xs px-2 py-0.5 rounded bg-zinc-700 hover:bg-zinc-600 text-zinc-300 hover:text-white transition-colors">
+          <button onclick="event.preventDefault();event.stopPropagation();window.open('{{.SourceURL}}','_blank','noopener,noreferrer')"
+             class="inline-flex items-center gap-1 text-xs px-2 py-0.5 rounded bg-zinc-700 hover:bg-zinc-600 text-zinc-300 hover:text-white transition-colors cursor-pointer border-0">
             <svg xmlns="http://www.w3.org/2000/svg" class="w-3 h-3" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2">
               <path stroke-linecap="round" stroke-linejoin="round" d="M13.5 6H5.25A2.25 2.25 0 003 8.25v10.5A2.25 2.25 0 005.25 21h10.5A2.25 2.25 0 0018 18.75V10.5m-10.5 6L21 3m0 0h-5.25M21 3v5.25" />
             </svg>
             Source
-          </a>
+          </button>
         </div>
         {{end}}
       </div>