perf(ui): increase viewport in gallery

This allows us to prefetch more images and reduce how often placeholders
are shown as we fetch more images in the gallery.

invokeai/frontend/web/src/features/gallery/components/NewGallery.tsx

@@ -511,7 +511,7 @@ export const NewGallery = memo(() => {
         ref={virtuosoRef}
         context={context}
         data={imageNames}
-        increaseViewportBy={2048}
+        increaseViewportBy={4096}
         itemContent={itemContent}
         computeItemKey={computeItemKey}
         components={components}