Text2Image carousel: cross-set navigation at arrow key boundaries

Instead of looping within the image set, pressing Right on the last
image advances to the next set and Left on the first image goes to
the previous set, consistent with Up/Down behavior.

frontend/libs/components/lightbox-modal/src/lib/lightbox-modal.tsx

@@ -334,7 +334,7 @@ export function LightboxModal({
   }, [batchImages, imageUrls, imageUrl]);
 
   const [selectedIndex, setSelectedIndex] = useState(0);
-  const carouselOptions: EmblaOptionsType = useMemo(() => ({ loop: true }), []);
+  const carouselOptions: EmblaOptionsType = useMemo(() => ({ loop: false }), []);
   const [emblaMainRef, emblaMainApi] = useEmblaCarousel(carouselOptions);
   const [emblaThumbsRef, emblaThumbsApi] = useEmblaCarousel({
     containScroll: "keepSnaps",
@@ -446,7 +446,7 @@ export function LightboxModal({
   );
 
   // Keyboard navigation:
-  //   Left / Right  →  carousel slide (loop wraps automatically)
+  //   Left / Right  →  carousel slide; at boundary, moves to prev/next bundle
   //   Up / Down     →  previous / next bundle
   useEffect(() => {
     if (!isOpen) return;
@@ -456,14 +456,14 @@ export function LightboxModal({
 
       if (e.key === "ArrowLeft") {
         e.preventDefault();
-        if (emblaMainApi && effectiveImageUrls.length > 1) {
+        if (emblaMainApi && effectiveImageUrls.length > 1 && emblaMainApi.canScrollPrev()) {
           emblaMainApi.scrollPrev(true);
         } else {
           onNavigatePrev?.();
         }
       } else if (e.key === "ArrowRight") {
         e.preventDefault();
-        if (emblaMainApi && effectiveImageUrls.length > 1) {
+        if (emblaMainApi && effectiveImageUrls.length > 1 && emblaMainApi.canScrollNext()) {
           emblaMainApi.scrollNext(true);
         } else {
           onNavigateNext?.();