Fix lightbox thumb strip not scrolling to current plate on direct load

lbBuildThumbs() was called after goToSlide(), so the scroll in
syncThumbs() ran against an empty container. Reorder so thumbs are
built before goToSlide() positions them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/js/app.js

@@ -220,9 +220,9 @@
     if (!lbBuilt) buildLbSlides();
     lb.dataset.open = 'true';
     document.body.style.overflow = 'hidden';
-    goToSlide(lbIdx, false);
     lbBuildMeta(lbList[lbIdx]);
     lbBuildThumbs();
+    goToSlide(lbIdx, false);
   }
 
   function lbClose() {