Add debug logging for thumbnail generation and improve error handling

dicom-viewer/src/App.tsx

@@ -437,6 +437,7 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
     }
     // avoid regenerating
     if (stackThumbnails[stackIdx]) {
+      console.debug('generateThumbnail: thumbnail already exists for', stackIdx);
       // already have it
       return;
     }
@@ -551,7 +552,9 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
       console.warn('Thumbnail generation failed for', chosenImageId, err);
       try { setStackThumbnails(prev => ({ ...prev, [stackIdx]: 'BROKEN' })); } catch (e) { /* ignore */ }
     } finally {
-      try { renderingEngine.disableElement(viewportId); } catch (e) { /* ignore */ }
+      try { renderingEngine.disableElement(viewportId); } catch (e) { 
+        console.warn('Failed to disable thumbnail element:', e);
+       }
       if (el && el.parentNode) el.parentNode.removeChild(el);
     }
   }, [container_id, stackThumbnails]);