feat(ui): add card image hover preview element to deckbuilder

Add #deck-card-preview div with img element that the existing
CardPreview JS object references. This was missing from the HTML.

ui/deckbuilder.html

@@ -476,5 +476,9 @@ Or plain list:
     </div><!-- /#deck-builder-page -->
 
     <script src="deckbuilder.js"></script>
+    <!-- Card Preview Popup (hover tooltip) -->
+    <div id="deck-card-preview" style="display:none; position:fixed; pointer-events:none; z-index:60;">
+        <img id="deck-card-preview-img" src="" alt="Card preview" style="width:220px; border-radius:8px; box-shadow:0 8px 24px rgba(0,0,0,0.7);" />
+    </div>
 </body>
 </html>